Why do parts of my windows blank out in KDE?

My computer: Lenovo ThinkPad E15 Gen 2 with Intel Corporation TigerLake GT2 [Iris Xe Graphics] (rev 01) graphics controller, running Debian 11 and KDE plasmashell 5.20.5. I have an odd problem: several windowed applications (konsole, firefox and others) 'blink' from time to time; in konsole if seems to be connected with text being printed - see the two pictures below. Sometimes a line 'disappears' after a line of text is printed by ping, then it comes back when the next line is printed. On other occasions it is a whole area of text, and ofcourse, it is intermittent. It doesn't seem to affect all programs, but I have certainly seen it in konsole and firefox. Any idea how to address this?