So are we all in agreement that Tin knows Phob is using him for information, he just loves him so much he just gives in. Tin is very smart and we can see it through Ongsa's incredible eye acting that he's suspicious by some of Phob's questions, but he can't deny him at this point. He has suffered through all their time apart–both from the heartbreak of losing someone he loves and from having to shove himself into a cookie cutter image of a prince–and he is in a place where he doesn't have any part of himself left to lose. Part of him, if not all of him, knows Phob is betraying the royal family with information from him, but he gives it anyway.
And if it's not all force of will, some of it is guilt. Tin feels he's stifled his success despite seeing Phob thrive against all odds. So he will give him things he needs to succeed, even if that's at his own detriment. Which is the crux of the plot, that Tin is willing to give and give and give, in whatever way that means in the moment–giving Phob release with that letter, giving Phob information, pushing him out the way of a gun. He'll keep giving til it kills him because it means Phob will get something positive out of it. He just really, truly doesn't see himself as a positive thing for Phob. He sees himself as selfish and pathetic and weak. He can't say no, can't stand his ground, can't keep up the walls, can't stop holding him close even if that puts him in the line of fire.
All of this when, really, they are meant to be two boys kissing in the open in a pile of flowers and confessing their love in the happiest moment of their lives.
