This is a bug related to taskbar pinning and isn’t a Nix issue. I’ve seen this bug on Plasma on other distros. I’ve even seen it on Windows once.
The bug comes from having originally right-click pinned icons from an expanded menu, instead of pinning them from a running program. You can fix this as follows:
- Close the affected programs and unpin the broken icons.
- Open the programs fully, so they’re running on your desktop. The icons will appear on the taskbar.
- Right-click the icons to pin them from the running programs.
Once you close these programs, the icon should still be correctly displayed on your taskbar.
Please try it as described above and hopefully that should fix it for you.