This is extremely counterintuitive. If you have an enslaved pawn, imprisoning them should not be considered an affront to their home faction.
Just saw this bug myself and turned a neutral faction I just traded with hostile when I wanted to arrest a slave to recruit him.
Steps to reproduce:
1. Purchase a slave from a slave trader.
2. In the Slave tab for the new slave, select Imprison.
3. A warden will come by and arrest the slave.
- Extra credit 1: The warden will insult the slave, get into a social fight, and need help from other nearby colonists to beat the slave down.
4. The faction the slave was purchased from turns hostile immediately when grabbed by the warden.
- Extra credit 2: The warden will place the downed slave on top of the bed instead of in the bed and come back later to move the slave into the bed.
I think this has something to do with the slave being tagged as lended because the faction the slave was purchased from was still shown in the Bio tab. Immediately after arresting, my faction no longer appears in the now former slave's Bio tab.
This bug (and the "extra credit" I noted above) appears fixed in 1.3.3074.
Thanks for the update.