Hi,
In my testing, 2 clean installs of LE9.0 on RPi don't automatically recognize a change from 16:9 to 4:3 aspect ratio and vice versa in "normal" view mode.
The same Kodi 18 on PC with same videos is OK.
There is a (somewhat tedious) workaround - stop playback after the aspect ratio change, then restart.
This happens e.g. on a 4:3 TV program with 16:9 commercials inserted.
- OE7 / Kodi 16.1 didn't have this issue