Window hint - set by program 'Sticky' - is ignored

[ALinuxUser]

I ran the program Sticky (version 1.2; for details of the program, see https://github.com/linuxmint/sticky). A window appeared and an icon appeared on Plank (version '0.11.89 RELEASE') and a tray icon appeared. So far, so good.

Upon closing the aforementioned window, the icon for Sticky persisted within Plank and indeed Plank marked the app as running. (See screenshot.) But - and this is the problem - clicking that icon, or scrolling the mouse wheel over that icon, did nothing.

I reported the problem to the author of Sticky (see https://github.com/linuxmint/sticky/issues/29#event-5045737406; indeed I adapt my text above from that bug report that I filled). That author says that Plank is ignoring a 'window hint' - a hint, I take it, that tells Plank not to show an icon for the program.

The same problem occurs occasionally with the program Rainlendar (https://www.rainlendar.net/cms/index.php), i.e. Plank shows an icon when it should not.

Linux Mint 20.2 x64 Cinnamon, which uses Mutter.
Plank version '0.11.89 RELEASE' - installed via PPA.