BS.Player version/build?
I understand you set "Aspect" under Options > Preferences > Playlist to "Original".
If you open
old style playlist there is another box (at top-right) where you can set "Aspect" separately for each individual movie in your playlist!
E.g. for the first file Aspect may be set at 4:3 and for the second file at 16:9 ... etc. These settings override the default setting under Preferences.
Perhaps that is happening to you?
(Of course you already know that you can save a specific playlist and its several settings to a playlistfile on you HD by rightclicking in playlist > Save to file > Text format .... etc)