As of 1.14 this still seems to happen and I didn't see any mention of it being fixed in 1.15.
Spent ages today searching for the final raider, then eventually we went back through a nearby Nether portal and found a raider pillager in there. We killed him in the nether, but that did not decrease the "1 remaining" count back at the village in the overworld so it just remained stuck like that.
Can confirm this happens on my multiplayer server (for all players on the server too) on 1.14.4