No, it's not the mod itself, or the item causing it. The custom themes aren't even the cause of it. It's the fact that the avatars are handled via a radio button that apparently requires an option be selected. Members have only the option of a server stored avatar in their profiles, and as a result it removes their custom avatar (bought in the shop) whenever another field is altered on the "Forum Profile Information" screen, because (no avatar) is automatically selected in the server stored avatars section.