Muxed the subtitles with the video stream into an mp4 and played the file with mplayer and ffplayer to compare and they both displayed the same white borders.
Uploaded the files (Test-AC3-5.1.avi, TITLE_1_0_lang_en.idx/.sub and muxed.mp4) I used to my
GoogleDrive as subtitleTest.7z
(no clue how to hardcode idx/sub subtitles with ffmpeg, but since ffplay displays the colors correctly on the muxed file I guess there is something wrong with the filter,...)
Cu Selur