So, as the title says, the items equipped on the Curios slots desynchronize from the server when the player rejoins, this leads to the items not appearing on the client-side, and being visible for other players, and still working as intended.
This happens until the Curios inventory is updated, leading to the client syncing again.

This can be really annoying in some cases, because even if the items still working as normally, if they require a confirmation from the client (such as Traveler's Backpack to open the backpack), they will stop working until the player update their inventory.

Game's information, in case this is required.

Minecraft 1.20.1
Forge 47.3.29
Curios API 5.11.1

Arclight Server 1.20.1 (1285 - 47.3.29)

Here's a video showcasing the bug

(Sorry for the low quality and framerate, my pc is really bad and can barely keep Minecraft)

Since this is being reported on an Arclight server, I cannot unfortunately debug this as given due to the nature of the hybrid environment. This bug would need to be reproduced on a purely Forge server in order to investigate further.

I've tested it on a Forge environment and this does not happen, seems to be a new problem from Arclight side (that possibly started on newer versions, since a year ago when I made a server using it didn't happen). I tested the software with other mods and seems that it doesn't sync data when joining as Forge does, leading to this and other mods desynchronization. I'll close this issue and report the problem to Arclight.