In "textmode" boot a large number of services chained from systemd "user.target" aren't ever started hence audio and other things aren't preconfigured properly for Kodi use. It's interesting to note a possible correlation with Kodi being started, but the assumption that "the hdmi audio device driver falsely grabs the sd-card interface" is creative thinking, but not possible in code.
Have you previously reimaged the box with one of the custom legacy-kernel images? or is/was it still running a factory image? If the former, does the problem exhibit if you restore the box to factory spec first?
You're right. The audio driver interface was a very rough guess
Trying to solve the 'dull colors on H.256 video' problem I have been trying a lot of various systems lately, including a late Android 7.1.2 version programmed to the flash which is currently running alongside with CoreELEC legacy 9.2.8 on the SD card. (Now I'm curious if I'm allowed to mention CoreELEC in this forum here
I'm not sure if it's possible to get the box back to factory spec again - at least not without overwhelming effort.
Since Computeraar's Minix seems to run the stock firmware in the flash this might not be te root cause of the problem.