Why are some documents not rotated automatically in FlexiLayout?
In case some documents are not rotated automatically in FlexiLayout, you can check the following settings:
Tools > Options > Orientation And Units
The above is the default variant of these settings.
If some of your documents are not rotated automatically, you can try to either turn the above settings ON if they're disabled, or turn them OFF, and check the results.
These settings are only applied to the project, not to the whole program, so you can have different autorotation settings project by project.