Since kubuntu 22.4 LTS (dolphin 23.08.5)
sometimes a folder, especially always the symbolic link to the user’s media mount folder, shows no subfolders in the left (directory tree) panel, although the right (file listing) panel shows the subfolders, even refresh menu button or f5 do not help. Only when the surrounding folder in the left tree is collapsed, then the refresh menu button or f5 is activated, and then the surrounding folder is uncollapsed again, the subfolders become visible in the tree.
Sorry, 22.4 was the previous kubuntu version, my problem appeared after upgrade to 24.04 LTS.