Pretty sure this isn't a Breeze icon issue. Please report this to the Calibre developers. Thanks!

I tried this, but got the response that this is an issue of Plasma or Qt. I would appreciate if you can advise where to request a fix.

https://bugs.launchpad.net/calibre/+bug/1784210

Hmm, OK. Re-opening then until we can figure out who's responsible. :)

It's of course still most likely a Calibre bug.

Where did you install it from so I can try the same version?

It is the Calibre 3.31.0 package of Arch Linux. There is a Calibre icon in the kicker menu, but not on the task bar or in the top left corner of the window decoration on Plasma Wayland.

Looks like Arch packaging problem - installed in neon on Wayland from https://calibre-ebook.com/download_linux and correct icons are displayed both in titlebar and in task manager

(In reply to Alexander Mentyu from comment #6)
> Looks like Arch packaging problem - installed in neon on Wayland from
> https://calibre-ebook.com/download_linux and correct icons are displayed
> both in titlebar and in task manager

calibre installed on neon according to instructions from your link runs under xWayland (X11), not natively under Wayland.
Calibre from Arch repos runs natively under Wayland.
I also installed calibre via flatpak/flathub on Arch, it shows correct icon because runs under xWayland.

On Arch Linux with Plasma 5.17 and Calibre 4.1 the icons are now visible on Wayland (not xwayland, confirmed via ALT+SPACE and then kwin, it is a wayland window). I could attach calibre with the icon as well to the task bar, so I consider this as fixed

Thanks for the update!