Ideas
I'm having this issue with newly spawned/generated NPCs, while previously existing ones are unaffected. I play with aging off, so I rarely lose sims because they don't die of old age. As a consequence, I have a lot of long time service sims, and I know the sims in my world. One of my bars recently lost a barkeeper to a weather related tragedy, so the game spawned a new one. He turned up wearing the barkeeper uniform with a bicycle helmet. All his previously existing colleagues are unaffected.
I also have a new community gardener wearing an astronaut helmet instead of the proper wide brimmed hat, while a previously existing community gardener has the correct hat on.
At the spa, played sims and sims that have been in the save for a while wear only the bathrobe, new sims that were recently spawned or created (and have thus never been to the spa before) wear random headwear with the robes.
The bug apparently happens on generation of new sim outfits, if a sim already has an outfit they seem to be okay? And "has an outfit" includes hidden situational outfits they have worn before.
My sim reached level 10 in the musician career, so she had worn the uniform (fancy black evening gown) before. When I had her reenter the musician career (so I could have her paint a self portrait in the uniform), she wore the musician uniform (the fancy black evening gown) with the astronaut helmet. Now she's going to work as an artist wearing the painter uniform with the astronaut helmet.