They're not outside because it's winter.Are your barn/coop doors open? They could be leaving their building and eating grass outside.
I have no grass on my farm. I also don't plant wheat the often.If let outside, animals will prefer consuming grass rather than hay from their barn/coop, one grass per animal. Also, note than animals are autofed by the game on festivals.
It may also possible that your hay income (through grass cut or wheat harvest) is greater than your animal consumption of hay resulting in a always full silo.
I think I'll try to remove the silo and then build it back.I have the same issue on android. I have not harvested hay in years. In winter silo does not drain.
I am not terribly upset with this turn of events, but I did want to chime in that it isn't just this one user.