the mod is installed on both client and server, getting a cosmetic and disconnecting from the game will remove it from existence

Could you provide versions of the mods? also Forge version, and if the server software is purely Forge or mixed?

im using CosmeticArmorReworked-1.19.2-v1a.jar in the following modpack: https://www.curseforge.com/minecraft/modpacks/better-forever

tested the void of the items on a server, using a turtle helmet and a sombrero from alexs mobs. it might be an incompatibility with another mod. As far as i know this is only a forge mod, idk what you mean by mixed.

I mean it if you are using a Forge + plugins software...

Does this also happen in Singleplayer in that modpack?

Edit: most mods in that modpack I have played with in my personal pack, didn't have problems. The only mod that touches saves I think is Smooth Chunk Save, Could you try without it? (Although I don't think it would affect player saves, just in case)

You could start a server on your local machine to test.
Do you have access to your server world files? to check if there are .cosarmor files inside "root/world/playerdata/".

nothing wrong on singleplayer and cant really test without smooth chunk since theres people playing on the server atm, also theres no plugins, but the server is running on docker

there are cosfile there, however i noticed something. Theres one that was updated, along with 2 other files, an hour ago, probably from a friend, then theres my files, normal inventory files say updated seconds ago, but the cosarmor file has not been updated in 5 days. ill try to delete it and regenerate it.

i have fixed it, this world was ported from singleplayer to multiplayer, early after switching i lost my inventory server side, so i decided to move my inventory from singleplayer into the server, for some reason cosmetic armor did not recognize my cosmetic armor file i imported from singleplayer and it also wouldnt write over it, never updating it. After deleting the file, equiping cosmetics and logging out, a new file was created and after logging back in my item did not disappear. sorry on my end. (this also shows cosmetic armor not overwriting / writing on previously created / imported files, this could be fixed but since its such a specific problem idk if its worth your time)

Hmm, It would be because it failed in loading/saving those data.
It should have logged some information in server logs, try searching "Failed to load CosmeticArmor data" and "Failed to save CosmeticArmor data".
Data from other saves generally are not compatible, because item ids are not same, it would need proper conversion to be able to use in another save.
Adding/Removing/Editing/Updating mods in the same save work because Forge keep tracks of old ids.