I was messing around with a pillager raid captain and my frost walker boots when the ice melted and the pillager fell into the water but couldn't get back onto the frosted ice. I tried searching this, but it's so niche, and I'm not sure what exactly is causing the issue, so it is still possible this is a duplicate. Either pillagers or other mobs struggle in water, or they struggle getting out onto frosted ice for some reason. I don't know whether it is related to frosted ice, or whether it is just pillagers or not.
My guess is that mobs have trouble climbing out of water onto [frosted] ice for some reason.
I've done some research, and the closest I could find to this is
MC-147844, which was fixed around 1.14 and regards pillagers and other illagers getting stuck in water. Possibly related to MC-104631, but the pillager is still pathfinding properly and is not rotating except when it looks at me while I move around (so normally).
However, it is definitely a cause of the blocks being ice. The pillager managed to get out on a corner, but then immediately slipped back into the water.