Game mode: [Online | Singleplayer]
I could reproduce the bug where thralls fall through floor and how to stop it.
If a thrall is in follow mode and cant follow you through a certain area of your base it will teleport somewhere else, after that this thrall will bug and always fall through foundations if picked up and replaced. Not only that the thrall will also ignore all structures, stairs, foundations, level differences, etc… when following you around.
The only way to stop this is place the thrall in a safe location outside, relog, run away from your base (outside render mode) and come back… the Thrall will be working again after this process