It's possible to gain strength in certain movements and lose strength/muscle in others. E.g. if you stopped doing upper body work completely, you'd probably lose a little upper body mass after a month or two, while your lower body strength/muscle might still go up.
Otherwise, I'd have a hard time seeing it as particularly possible/plausible. Hydration and such can have a pretty marked impact on how muscles "look," so sometimes people think they're losing muscle when their muscles become somewhat depleted of water/glycogen, when they're really not.
Stated differently, any training that results in a gain in strength is, probably at worst, going to have a neutral effect on muscle mass. I.e. it's possible to get stronger WITHOUT getting more muscular (neurological efficiency and all that jazz), but stronger while LOSING muscle? Less likely.