Can ACINQ (Phoenix wallet peer) steal my funds during a recovery?

Nothing.
If you have lost the last state and are recovering it from your peer then you are trusting it, no matter if it's encrypted or not.
They could lie and continue operating (or close) from a state of the channel in which they had a higher balance, or could serve you an encrypted backup for a revoked state and trick you into force-closing to steal your funds through the penalty transaction.