Using the ears skin manipulator I made this skin:

But in-game a black bar appears on the back of the skin.

How to fix?

If you have First Person Model installed, it is incompatible with Ears

oh

To be fair, it was attempted to be fixed some time ago, but it seems to have cropped up again as an issue. I don't know if its 1.17 specific or not that its an issue, as that was the only time I've attempted to use First Person Model with Ears.