The stack is a part of the inventory system, not an array. Its a Lyra thing.
The issue is resolved… what appears to have happened is the inventory system I am using (ArcInventoy) changed a variable in one of their classes from TObjPtr to TSoftObjPtr which then would no longer trigger replication on the server. So it wasnt an unreal 5.4 to 5.5 thing, it was a plugin update that I did at the same time… lesson learned to do them separately lol