This question already has an answer here:
The current handling of bounties when questions are migrated is somewhat inconsistent and incoherent.
Already awarded bounties are not handled as consistently, and bountiers complain about it on meta:
In this recent case, the points were simply disappeared, gone from both the offerer and the earner on both the source and destination site.
In this earlier case, the bounty was initially deducted from the earner and refunded to the offerer, but then a mod stepped in to restore the bounty as previously offered and earned.
This is only going to come up more often as the SE network expands, especially as the new
/review system facilitates migrating old flagged questions which would otherwise have just lingered.