Did you not notice the update to Plasma 6?
This is a major update.
Are you sure all your resources like theme and icons are up to date and compatible?
Its also a good idea to refresh the configs and cache as shown here:
https://wiki.archlinux.org/title/KDE#Configuration_related
It is also worth noting that in Plasma 6 your Icon theme determines your icons in many areas previously defined by, ex, your desktop style. And icon paths have changed in some places so you need to use an up to date icon theme.
Here is mine if you want it:
MACE / Icons / Papirus Mace Icon Theme · GitLab
Its never a bad idea to take a look at the update announcement thread either, especially the ‘Known Issues and Solutions’ second post.