Can treated syphyllis come back?

My two were treated for syphyllis not all that long ago, one now has a crusty-ish area to his top lip area. Not sure whether this is something else, an injury or something, or can the syphyllis come back?

Yes sometimes it can - best get back to the vets for them to confirm, and get it treated again. I have a feeling that in some buns it doesn't get completely eradicated but can be like cold sores and lurk waiting to come back at a later date. It might be worth discussing with your vet whether a longer course would help try and prevent a further recurrence?

My Tinkerbell had it treated but it returned about 3 months later.

She was treated again, with 3 doses of Penicillin, 2 years ago, and it hasn't returned, thank goodness!