Yes, the day I was trying the plugin with Plasma 5.18 I recompiled and reinstalled it. Though, today I've updated to plasma 5.18.1, recompiled and reinstalled ShapeCorners one more time and now it works. So maybe the problem was in kwin or something. Anyway, everything works and this ticket can be closed now. Thank you.
Doesn't work with Plasma 5.18
Yoshimi LV2 crashes in Ardour 4.2