Precedent for creature tokens to exist in the hand without ceasing to exist?
A token cannot exist in your hand (or anywhere else other than the battlefield) long enough for anyone to do anything with it. It will cease to exist as a state-based action before anyone gets priority:


State-Based Actions


704.5d If a token is in a zone other than the battlefield, it ceases to exist.


Mutate does not affect this in any way. Once the mutated creature leaves the battlefield; each individual card or token is placed in the appropriate place, so you will have a token in your hand that will cease to exist.

721.3. If a merged permanent leaves the battlefield, one permanent leaves the battlefield and each of the individual components are put into the appropriate zone.

It’s possible that there’s a bug in MTGA with not handling mutate properly; but I can’t say for sure why it looked like the token was in your opponent’s hand.