Okay so. Alastor spent some time in Vox's captivity, didn't he? And Vox definitely used electrocution on him. Probably more times than we saw on screen. Because you can't just be a walking taser and not use it, right? And one of the most common consequences of electrical trauma is arrhythmia.
So Alastor is stuck with it. Perhaps temporarily—as in for weeks or months—until regeneration slowly does its job. Perhaps permanently or semi-permanently—as in healing it would require full body regeneration, and he isn't going to die, even if for a short time. He won't be able to explain his presumed suicide to Charlie. And he isn't allowed to leave the hotel for time it takes to die and recover because "mhm, Alastor last time you walked out on us Vox declared the war to Heaven and almost exploded Hell, you're not leaving anywhere. You can take your vacation next year and you must notify us beforehand so we would be able to barricade the doors."
Anyway, Alastor suffers silently (again) and doesn't know when and if it ends (again!) and maybe doesn't even completely understand what is happening to his body (again!!!). He is unpleasantly reminded of his time with the angelic wound. Just when he thought he was finally rid of it and could return to his usual life!
Alastor is so pissed. He put up with Vox and all his bullshit, he broke two deals that tied him up in five minutes, he technically prevented a war with Heaven, he forced Rosie to fix his staff, he kept Hell from exploding (along with others, but still!), he is free, he is on the top again, his game is continuing—
And his stupid body refuses to cooperate again?
And this time it isn't because of a wound inflicted by an archangel in battle. This time it's because of something Vox did to him. In captivity. To which Alastor surrendered himself willingly.
He feels humiliated. He feels violated—because he doesn't want to let Vox influence him in any way. To leave any trace on him.
(He knows Vox would get off on the thought that he caused long-lasted damage to Alastor, and it makes everything so much worse).
Besides all these mental struggles, there are physical ones. Alastor feels his heart trying to break through his ribs. He feels it stutters. He gets out of breathe easily. His chest aches. He is even more tired than usual. He feels lightheaded. He nearly faints sometimes—or actually faints. Maybe he barely manages to hide from hazbins before collapsing.
And these arrhythmia attacks are sudden. They often catch him off guard. Of course, physical and mental stress triggers it, but some of them are completely random. He can sit there and do nothing and still nearly faint.
Does Alastor even understand what it is? He may write it off as panic attacks. There are similarities, after all. In this case, he tries to repress his emotions further, mentally screams at himself for not being able to just "get over" everything he went through while being captive. He keeps himself busy with work at the hotel and scheming, and it only makes arrhythmia worse.
Maybe he catches that it's something physical—but what he can do about it? Besides his dubious attemts to self-medicate, he probably tries to just wait it off.
In any case, Alastor is even less inclined to ask for help than with angelic wound.
If he might entertain some thoughts that hazbins would care about him before, now he knows they wouldn't.
He has seen Lucifer trying to pick into his insecurities, and nobody interfered. He has seen nobody came to save him when he was captive. He has seen nobody cared when he was injured by Shock.wav. He may even hear about Charlie's complaints how inconvenient it was that he got himself captured.
Alastor is completely sure that they would ignore it or treat it as another inconvenience. Or write it off as him scheming again. Or pick at it and use it to hurt him, in Lucifer's case.
Besides that, he knows now how sloppy they are with their secrets. With any secrets, actually. Lucifer said that he can't hurt sinners once!—and it landed straight on the Vox's ear. Yes, Angel is not here anymore—but there are dozens of sinners in the hotel now, and most of them probably have contracts with Hell-knows-who. Alastor can't risk to expose his vulnerability like this.
And also? The last time he was trying to ask for help? With Rosie? It went awful. He got slapped in the face with "do it on your own" and it pissed him so much that he broke free just to rub it in her face. The very thing he is suffering from now happened because of it. He just isn't ready to take this risk again.
So he'd rather literally fall dead than admit that something is wrong with him.
Will anybody notice that something is off about Alastor? Will they try to do something or just brush it off as him being difficult again?
I think Lucifer and/or Emily could notice. Charlie and Vaggi are too busy with new guests, Husk is going through his own gay drama, and Niffty is nice but totally uncritical about anything related to Alastor.
Lucifer may notice because he was captured by Vox and suffered as well (and was left alone after the final song—), so he may feel some kind of bond with Alastor. Plus they are going to interact more after season 2 ending, so they may start to—and Lucifer may notice that something is off.
Emily is very caring and the most unbiased—she doesn't have this "Alastor being Alastor" prejudice. He is just a random soul for her, a friend of her friends, and she can clearly see him suffering! Maybe she can even feel it because she is the Seraphim of Joy, so she may be extremely perceptive to other's condition.
I love this whole idea - I especially love it being Emily! And she can end up telling Charlie and the others, Charlie confronts Alastor on why he didn't tell her, and he can tell her all those reasons why - especially if he's RIGHT and some sinner finds out and and the news gets out - Vox has mixed feelings.
On the one hand, he marked and hurt Alastor! But he wasn't trying to. He - he didn't mean to, Alastor, please -
(He could have died and Vox wouldn't have known. Vox could have killed him - and been left in a hell without Alastor.)
Vox doesn't want to kill Alastor he wants Alastor's approval and respect and affection and trust. He wants him at his side and kneeling at his feet. He wants to top him. He wants to dom him. he wants to sugar daddy him. He wants to hurt him.
Well he did. Are you happy now, Vincent?
(Also this means Emily saved him AGAIN!)