Hi all,
Ran into this (supposedly) bug some days ago. Due to a planet being highly contested during a war (I can remember the planet was invaded by both parties several times), it ended up with Enfi colonists.
As Enfi can't be freed, being servile, they can't be turned back into slaves neither.
Though it was a bit annoying (split populations, Enfi were eligible for the colonize planet build, and were able to board colony ships), nothing really serious stemmed from this.