It seems the packaging problem in the Mandriva package (documented here and here) has also occured in Ubuntu Studio 25.04 - i.e. that the brush assets are missing. The poster of the Mandriva bug report suggested downloading the Windows version and copying the assets from the .zip file.
My solution was to download the Linux executable 4.4 version from the Blender website. So far that is working fine but I’ve only had time to test it briefly.
As this is a support/help issue, I have moved it to the correct category.
As Ubuntu gets that package unmodified from Debian, I took a look and found out it’s a known bug.
Give it time and patience and it will get resolved. For what it’s worth, you can always look at the known bugs here and here.
For what it’s worth, as it’s a known bug, there’s no need to file or report it.
Additionally, since this is a discussion about a known bug, I’m going to go ahead and close this topic to make sure the discussion about the bug happens on the respective bug tracker(s).