Hey there bud, so the item being removed is equipable by every class it is a sort of an accessory item but it has no stats. It's equipment type id is ten but there are a total of 11 equipment types currently.
I believe it has to be the other situation you listed. It seems that it only occurs where there are a different amount of equip slots so yeah just like you outlined with the Class a and class b portion.
I currently have 17 classes it can change between and nearly each one has a different amount of equip slots so it seems like this might be an issue.