I have written down two fixes i have found to said issue.
The first one has the highest success rate.
The fix that seems to work the most is to change the default playermodel that you’re using, this means one the four default playermodels that you can pick in the appearence menu.
So for example if you’re wearing the milk carton playermodel as default, change to the skeleton, male or female playermodel.
The following fix doesn’t always seem to work and should only be done if the first one failed.
Another fix that sometimes seem to work is to momentarily disable the steam automatic cloud synchronization in the steam settings, then go into the steam folder in file explorer and navigate over to the folder called “ClientPlayerData2”
This is the filepath from Steam to said folder:
Once you have gotten into said folder delete everything inside the folder, wait a bit and then re-enable the steam cloud synchronization and start the game up.
The game should say that the files on your computer and the steam cloud isn’t synchronized and will ask you if you wanna download the files from the cloud or upload the ones you have locally, click on the “Upload to the steam cloud” button.
Now you’re done and can let the game load up fully where upon you should be able to select a workshop model from the appearence menu without having it disappear whenever you join a server or gamemode
NOTE: Certain files within the ClientPlayerData2 folder has information on what appearence items you’re wearing and what items you have equipped in your hotbar, so by doing this the game also forgets what you were wearing and holding in your hotbar.