EDIT: found out I didn't understand how UUIDLeast and UUIDMost worked. I thought it picked a random unused value between the two, rather than referring to most and least significant bits. Ignore this ticket.

If you have two identical pieces of armor, each with the same attribute modifiers, and then you wear one and hold the other in your hand, upon either holding something else or taking the armor off the attribute modifier disappears, despite still coming from the other piece of armor.