[Milhouse]

Here is the first Debug log as requested with Kodi set to 108060P
 http://ix.io/18xX

Here is the link after reboot which made it switch back to 4k30P

http://ix.io/18xX

The links looks the same?

I followed your steps

Ah yes... step 5 is incorrect and there needs to be a step 6:

5. Assuming Kodi starts in 4K/30Hz, login with ssh and run systemctl stop kodi (this flushes any new/modified settings to disk)
6. Run cat /storage/.kodi/userdata/guisettings.xml | pastebinit and paste the link.

I'm assuming there will be a difference between the two guisettings.xml, although what that would mean I'm not entirely sure right now...