OK, rough notes up, otherwise it'll take forever...
If a Wizard was used, which one, and which pack?
Fixes:
Auto switch DEFINITELY turned off in Android's settings?
Kodi and DBMC's display rate auto-change , not resolution I appreciate, but try flipping from current state to opposite. System (click), Video, Playback
Wiped Kodi/DBMC data and reloaded add-ons?
Tried OTA Update from last weekend, with latest 16.1 Stable (Kodi) and 16.2-1 (DBMC) to see if behaviour remains the same.
AdvancedSettings.xml If you can navigate to /Android/data/org.xbmc.kodi/files/.kodi/userdata after a reboot, before opening Kodi (need to check path is correct and find DBMC's to quote), rename the XML file to a new name. Open Kodi/DBMC, see if 4K 1080 switching occurs still . ES File Explorer/File Manager HD/preferred app is fine.
If you set to 1080p in Android, then play a 4K video in Kodi/DBMC, does the reverse behaviour happen?