If I can, I will.
I have an idea how we could potentially BAND-AID the issue if a BUGFIX remains elusive, but I can't make any promises yet because I haven't dug into it yet.
But... I did fix our broken spines (Update 7/23/2024) in SEVEN (7) DAYS, and I did BAND-AID the fake sleep glitch after WooHoo (Update 6/3/2020), and I did make a NudePart Override for the underwear glitch (Update 10/22/2024) with functioning reactions, and I am wrapping up an EnterBathing Override that's meant to supersede the NudePart Override (Update 10/22/2024) as "the last mod you'll ever need for all your Nude/Bathing Outfit woes" - don't quote me on my self-quote though! 😹
So yeah, no promises, but I'll look into it.
What I can add right now is that I've verified it happens whether the sim is clothed or nude when sunbathing, and also, has anyone not on console tried deleting their localthumbcache.package after this happens?
From my "BUGWATCH" write-up for this to remind myself to look into it:
TL;DR - Delete your localthumbcache.package either after your sunbathing session or after the buff expires and your skintone goes back to its regular hue.
Because this is an issue with the SimBust and not the sim (so, unlike what was happening with the Goth Galore Kit hoodie), you should not need to delete your outfit to get rid of the problem. You should just need to delete your localthumbcache.package file.
Unfortunately, if this works, it will only work for non-console simmers, because console simmers can't delete that file easily.
SimBusts are cached images. If you ever took a gander at AnimXGenWear, one of my modules changes the Flirty SimBust, and I said that you need to delete your localthumbcache.package whenever you enable or disable that specific module, because the SimBust is saved in the localthumbcache.package, and if you don't delete it, you won't see the new Flirty SimBust.
I haven't tested this fully yet, but I believe that all you need to do is SAVE or SAVE AS... your game after sunbathing, then delete your localthumbcache.package, and on next resume, your SimBust should look fine, because the game will regenerate it. You MIGHT need to wait til the sunbathing buff is gone and your skintone is no longer tanned, but it should work.
I'll verify it later.