So, I'm getting this warn on gamemode change link when custom items presents with NBT components from 1.21.4 (1.21.2+) as "equipment" e.g.

These items are base from ItemsAdder to get textured item with NBT components and edited with EcoArmor to get some special effects on armor etc which are packet-side.

Tested v5 from Discord and still the same https://paste.crafttale.eu/view.php?id=67e44f96d39ff

Have you contacted EcoArmor support on it?

Also can you link me to the plugin? If it’s free and open source I can test on my side as well.

Hey there, EcoArmor is open-sourced but not free.
ItemsAdder is neither one.

I've had same problem before with AuctionHouse plugin, where dev utilized to fix it.

I can pretty sure ASK if I can share a JAR for dev purposes.

This was the issue on side of AH, but I don't know if it's 100% same or not.

So is he trying to say EcoArmor serialisation code having issues or multiverse inventories?

MVI may be utilizing old serialization which can cause this but as I've said, not 100% sure if it is the same.

It's not old, they have to comply with bukkit ConfigurationSerialiable or not we can't do much

I've said may be using, not it's using as I don't exactly know how MVI works with MVC.

On the other hand, no issue was in 1.21.1, this started doing since 1.21.2 update.

Today I'll do some tests if it's done only for packet-based lore type or for all of the items, EcoArmor/Items created.

Wrote u a DM on Discord so we can communicate much better about this, before doing verdicts about it.

This is a server software bug, I have opened a pull request on paper to fix this issue: PaperMC/Paper#12352

New NBT serialization from 5d4fbd1 works!