After this weak updates in Kde Neon some things got buggy concerning the icon theme (I'm using Oxygen theme all over my Plasma workspace)... Here is an example (weird icons not matching the theme surrounded in red): https://i.imgur.com/OwsshTD.jpg . What is going on? Should I report a bug?

I noticed an icon mix up on my system too after the upgrade. Somehow a few bright, low detail (and ugly) icons from some other theme wound up on my Oxygen workspace. They stuck out like sore thumbs when amidst the more sedate and finely detailed Oxygen icons.

On the folder that's changed on your system... Try right clicking on it. In the window that opens, choose 'Properties'. When the property window opens, you'll see an image of the icon on the top left of the General tab. Left click on the icon image and you'll get a window showing a variety of icons for that file type. You should find one you like. Select that one and you're done.

For other file types.. Like .mp3 you need to go into the 'file associations' part of System Settings. Find the .mp3 association. There you'll be able to left click on the icon that's associated with that file type and find the correct Oxygen icon.

My .mp3 files all turned to these horrendous low detail florescent magenta things.. Had to go in to file associations to change back.

I don't want to do manual fixes, I want it to work as it should...

On the positive side: the bug is already reported, and it would seem that they are working on it: https://bugs.kde.org/show_bug.cgi?id=445804