I have found an issue with GBM, confirmed with both Generic and Amlogic, when configured as limited colour range. The issue being that images lines for dvb subtitles have the range conversion applied twice. In other words, the lines where dvb_subtitles are rendered, not just the text but the whole width of the screen appears lighter then the rest of the screen.
This does not occur with Generic-Legacy (X11) nor Kodi (X11) on Ubuntu 22.04 (x64).
I can provide a kodi debug log, however I'm not sure it would help.