Since you are in singleplayer on PC, I would suggest the following solution -
step 1 - make a backup of your current save
step 2 - in server settings , make yourself admin, open admin panel and activate god mode (or just ghost mode should also work)
step 3 - go to where they appear on the map and search below the ground (and, if necessary, up in the sky) - assuming you find them, select to move them to a new guarding position and try to place them back above ground
step 4 - if you don’t find them, just use the rescue function and then use the admin panel to spawn in the lost equipment
(step 5 - turn off god mode, then either reload the game, or open the command line (press tilde key - the key between tab and escape) and type ‘MakeMeNormal’ (with no spaces) to turn off admin mode)
I hope that helps - I can explain anything that is unclear.
Note: you are absolutely correct to make a bug report - this should not happen - but if you do not want to lose the thralls/equipement, this is the way to resolve the issue from your end, since Funcom will not be able to reach into your save and undo the bug, even if they are able to fix whatever caused it to prevent it happening again.
Not that I’m aware of, unfortunately. Best I can suggest is to use the ‘rescue’ function and then just spawn in the lost items (so long as you know what he was equipped with).