I think the change happened after I experimented with the “Titlebar Buttons” in Colors & Themes >> Window Decorations >> Configure Titlebar Buttons.
The problem is that adding the “Application Menu” has killed the menu bar completely, including on GTK applications, which don’t receive the “Application Menu” in their title bar at all, thereby paralyzing them.
Taking the “Application Menu” out does not return the old menu bar.
Here is one of many posts from the Internet on the issue. Embarrassingly for KDE, it’s over six years old.
I had a similar issue, don’t know why though, because I haven’t messed with window decorations nor global menus. Only this solution helped. Now I’m hoping it won’t return again.