Running ffmpeg WITHOUT editing path variable
I solved it. Silly me.

Turns out you have to use quotation marks around the file path.

"%~dp0\bin\ffmpeg.exe" -i "%%a" -map 0:s:0 -y "Temp.ass"


not

%~dp0\bin\ffmpeg.exe -i "%%a" -map 0:s:0 -y "Temp.ass"