Turns out this was a series of hacks breaking down in the floatbob FOF thinker. Apparently, if a crumbling + floatbob FOF fell down too fast, specifically falling faster than the FOF's own height, it desynced its own height due to its top height being unable to lower below the FOF's bottom height. This in turn somehow made P_FloorzAtPos return the FOF's own top height from then on, which caused the FOF's bottom height to then forever chase its own top height into space!
Stupid FOF dogs chasing their own tails, when will they ever learn.