In certain maps, mirage, dust2 and some other official maps, I always lose all my bet even if the system told me that I won, not sure about anyone else since I dont see other people complain about it.
It really depends. If you bet at the wrong time, like at the last second and the round ends either by time running out or a specific side wins the round, you may lose your money completely. The bet carries over to the next round and the outcome is decided then.
Best thing to do is bind your bets to a couple of keys.
Still get caught betting right before end of the round, which gets carried over.
On a side note, would be nice if more people dropped weapons when teammates are using pistols only.