Very possible a Leo will slow down it's eating during Winter months....and maybe even go off food completely in the spring (March - June)....but they should regain their appetite and go back to voracious little buggers when temps warm.
One of my girls never goes off food....my other slows down in the Winter, and then does a feeding strike in the spring. In fact, last year she took 13 weeks off, ate 1 meal worm, and took another 2 weeks off before her feeding response kicked back in. She's done that before so I wasn't worried, but never for that long.
The mouth scab thing is concerning though, and the fact that your other LG passed away at an early age (8 is pretty young for a LG) could also be a factor. I'm also wondering if removing the scab is the right thing to do? if it's really a scab, maybe let it heal completely and fall off of it's own...removing it may cause it to scab over again???? I never heard of a LG shedding the skin on the roof of it's mouth.