I had this problem this days, and i think i found a co-relation with the “must be placed on a walkable floor” i was having problems to place a thrall in just plain ground near my purge hut, them i runned around the area with the building hammer ready to place a foundation, i was looking if there was some claim or restriction to build, after i did it i could place the thrall in the exact same spot it was giviing error before. So faceing the problem of purge not spawning with a huge plain area ahead to it spawn i tried to do the same and run the area with the building hammer in hand ready to place a foundation. And it worked ! i do around the little pruge base, time i will do purges, just one time is needed after they do a first spawn the problem will not occurs unless you make some building change. Far far far of and ideal solution but is the workaround i found to mitigate the problem. Having the purge spawned you will find more bugs.