Equipping any hat causes the head to vanish. It only reappears once the hat is removed.
EDIT: After looking into it I found that this happens because your headbase_hat files are wrong. You need to relabel the top hat as "hair" and have another version of your existing headbase files that are edited to be compatible with hats as headbase_hat. This will fixed the head not showing up when equipping a hat and will also fix the top hat not showing up.