I have 'Reddington Shores' which I think is very similar. Mine has done the same thing. I think this is normal. I have had mine 3 years now without any big problems. I did lose another 'Reddington Shores' probably because it was in the same pot with a very aggressive begonia. Live and learn.

The correct spelling is 'Wimauma'. It's a Greg Sytch hybrid, one of a series named for places in Florida. Rhizomatous begonias go through a period of semi-dormancy in winter and often lose leaves. As long as the rhizomes are still solid, without any rot, they should come back in the spring. They won't require much water until active growth restarts, so keep them on the dry side.