Hello again.
I noticed an annoying and very weird behavior when Dolphin is emptying the trash. About 50% of the time it hangs when I click on "empty trash", the issue seems more likely if there is more than one item to delete. The only solution is to log out and retry or to delete the whole Trash folder in /.local/share. I'm running Plasma 5.18.5, Framework 5.74.0 and Qt 5.12.9.

Yes... I've noted the same!
after I click the empty trash button...
I've discovered rather than waiting for trash to empty
I just close Dolphin and then start it (again)
given my current setup, I see that trash is then empty
for now, it's a schlep...
but I'm trusting that this will eventually go back to that "instant" action

And so it has... as of today, at least with my config, trash behaves again as expected!

The issue was actually my distro. switching to another one fixed the issue. Problem solved.