When right clicking, open with, other a .cfg file, in the open with window, there is an option to remember this association for whatever is chosen. When doing the same with a .mp4a file, no remember this choice option is given.
Are you talking about .m4a files? If so, I can't confirm the bug.
An option to remember the file association is available when I right click on a m4a file and select "Open with > Other application..."
Operating System: Arch Linux
KDE Plasma Version: 5.15.3
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2