The bottom line is that when you make a panel opaque, then its elements become opaque, which seems to be the way it should be, but if you restart the computer, the panel remains opaque, but its elements turn out to be transparent. I've already gone over my head trying to achieve full opacity of the panel and its elements, but it didn't work out for me. That's why it could very well be a bug.

Stages of reproducing the problem.
1. First, make sure that you are using adaptive panel transparency.
2. Go to edit mode and in the advanced panel settings, set the "Opacity" parameter.
3. Check all the various elements in the panel. Application menu, volume, networks, system tray, and digital clock for opacity operation.
4. Restart the computer.
5. Check the given panel transparency setting and make sure it is "Opacity".
6. Check the previously listed panel items.
7. Conclude on the problem.

Recommendations

Make yourself a pizza.

SOFTWARE/OS VERSIONS
Linux: openSUSE Tumbleweed
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.2