I'm not sure if it's been fixed or noticed but I didn't see it
Here's a clip of it : https://medal.tv/games/minecraft/clips/lS1fkDUt6j1qH/d1337zBV1r7c?invite=cr-MSxidkYsNTI2NjYwMTUs
(Possession Detection was set to normal as you saw in the chat)
also /enchant doesn't work but I'm too lazy to get a clip of that

Edit : I didn't notice but after reviewing the clip again I didn't do any damage, its like the game's pretending I'm still a soul, and when I rejoin I got targeted by the wither
Clip of that : https://medal.tv/games/minecraft/clips/lS6YjkbwdgV3U/d1337ZnP4eXb?invite=cr-MSxFSkYsNTI2NjYwMTUs
Edit 2 : Its definetly thinking I'm a soul, I can fly freely and not take fall damage, also it turns out I do damage, but its just not appearing on boss bar (when the wither died the boss bar didn't drop or change colour or anything)
No Fall and Flying : https://medal.tv/games/minecraft/clips/lScx4e8lnNxU6/d1337P7eKXLP?invite=cr-MSxBUmYsNTI2NjYwMTUs
Damage : https://medal.tv/games/minecraft/clips/lS9YrG7Uxs8f_/d1337u5z8nNU?invite=cr-MSxRMnIsNTI2NjYwMTUs

specifically happens when using the command in spirit form. using the command in human form works fine
Tho I need to test it on 1.18.2 now.

Will test on the latest beta.

It happens when using the command as wandering spirit, it doesn't occur when you are remnant, no matter in spirit form or not.

Can confirm, still present in Beta 9. Creating a shell while in spirit form causes that shell to be invincible and unnoticed by mobs.

It also occurs when using the command while possessing a mob. This was tested without Pandemonium so it is definitely not Pandemonium exclusive.

After some testing, it seems there are some circumstances in which one of these "phantom shells" as I'll call them can become vulnerable to damage. One of these circumstances is when they're unpossessed and attacked by another player, but further testing will be required to determine if there are others.