Slams hands on desk. Does siffrin change into a dragon during the loops. Do they have to hide changes from their family. Do they
YES YES YES to all of this!
The loops are why Siffrin starts changing into a dragon in the first place (in addition to their fixation on their family - hoarding them, not wanting to lose them). They already have some latent draconic traits just due to being from the Island to the North, but the immense power of Wish Craft coaxes those traits out and grows them more. Dragons are powerful beings, after all. So the more loops they go through, the further they change.
(This is the reason loop is a full on dragon instead of arguably human with obvious draconic traits like siffrin is - not only were they in the loops for much longer than siffrin, but they also made that last wish at the end, finalizing their transformation)
Of course, Siffrin being Siffrin, they ABSOLUTELY do not want anyone to notice they are literally going through what is basically nonconsensual body modification, so they hide it as much as they can. Scales, already hidden under clothes, aren't mentioned, claws are hidden by gloves, teeth are hidden by simply not talking much or opening their mouth very widely, horns are hidden by hat.
The most trouble they have is with their eye. Their pupil becomes slitted and it's a difficult thing to hide. They try with their hat and their hair but that comes with the issue of either 1) being noticed for acting weird, or 2) not being able to hecking SEE.
. . . Granted, later on, they've gone through so many loops they hardly even need to see to know what they're doing.
Usually their go-to for their eye is to just ignore it/deflect whenever someone mentions it. One thing being weird about them isn't usually enough for them to be confronted on it.
(Of course the stranger they act the more suspicious they are the more others notice - the dragon thing IS involved in the Sus Quest and in fact contributes to odile's suspicions considering legends of dragons are mentioned alongside time and wish craft. But that can be a different post, should anyone want me to elaborate.)
Things come to light in Act 5/6 along with everything else, of course (especially considering that final fight is when their wings finally emerge), and now everyone has to navigate what it means to be hoarded by a dragon, and Siffrin has to navigate how to not let their fixation become all-consuming again, making sure their family keeps their autonomy. In addition to, you know, trying to actually talk about their feelings.
Throughout their post-game travels, they all try to get their hands on as many books about dragons as they can, so everyone knows how to take care of their little dragonlet and so he can learn how to take care of himself.