Try this on 5.19 versions.
This can be replicated as described on 5.19b.
The Master shows up at the top of the inventory/equipment roster until I try to modify their eyes in the lab. Afterwards they never re-appear in the inventory roster. (At least for the ~10+ days I checked)
Note that this was using my save from 5.18d. Not quite ready to start a whole new campaign just to test this one issue = P