I removed all the costumes of the slave in single player mode. After exiting the game, the costume appeared again, but there was nothing in the device bar.
this is a known bug, it’s so old a bug I’m surprised it’s not been fixed.
thanks for the reminder to the devs. 
a workaround fix is to give the thrall clothes, then remove them again.
but until it’s fixed you’ll need to do it every time you log in and out. 
This has cropped up from time-to-time. The previous solution was the following:
- Interact with Thrall and select “Follow”
- Modify armor components
- Interact with Thrall and select “Stop Following”
That’s too much trouble
But I still don’t understand
Okay, first you go to a Thrall and press E and tell them to follow you.
Next, you give them clothes, then take them away.
Finally, you press E and select Stop Following.
Now the clothes will not respawn on the Thrall.