Is it possible that you installed a non-stock task switcher (which handles Alt+Tab) back when you were using Debian 12, and it's not compatible with Plasma 6? T

Is it possible that you installed a non-stock task switcher (which handles Alt+Tab) back when you were using Debian 12, and it's not compatible with Plasma 6? That happened to me. The fix was simply to replace my old alternative task switcher with a current one. In case you don't like the stock one, there's a simple and clean one in the store called "Aqua medium icons".