As described in the KDE forum (https://forum.kde.org/viewtopic.php?t=141296):
I've recently upgraded to Plasma 5.10.4 and I'm having an incredibly annoying
issue. Occasionally, ALT-TAB is ignored the first 1, 2 up to 5 or so times, and
then it finally works. This is unacceptable because when I'm programming /
debugging / testing I have to switch windows and do a lot of different things
and I don't have time to wait for visual confirmation that my keyboard command
was acknowledged. I just send keystrokes to my hands and expect the computer to
work, but end up typing into the wrong window because of this.