I can confirm this - happens on both X and Wayland.

@zmike : Thanks - vertical ibar working again (E21.5).

I have this same problem and would like to add:
The vertical shelf need not contain only the ibar.
The height is not zero but maybe one or two pixels high, so it is possible to hover the pointer over the ibar, grab an pulsating icon, move it, and then the ibar miraculously appears (see also T4621). When it does, it is obvious that the ibar is slightly too small - the top icon is cut off slightly on its top and the bottom icon is cut off slightly on its bottom. (On a horizontal shelf this is also evident - the lefthand icon is cut off slightly on its left and the righthand icon is cut off slightly on its right.) So the problem may not be the icon sizes, but rather the calculated ibar size.