If you do not gain experience, you do not lose experience. If your skill is maxed at 100.00 in the class you are currently using, you will never lose experience in another craft when desynthing. I'll put that in the OP. Even if you could gain experience, if you didn't gain any, you can't lose any.

HOWEVER, there is one exception to this: desynthing while over the 300 point limit. If you're at 300.12 and you desynth anything, even if it gives no experience, you will lose 0.12 in a random craft that you have at least 1.01 in (unless every single desynth class you have points in is "locked" but playing with that is really irritating if you desynth hundreds of items daily like I do). If that random craft hits 1.00, it won't go below 1.00, and if you're still over the limit, your next desynth will trigger this again and so on.