Reverse field order only for part of the video using ffmpeg

[Compact]

Hello, I'm new here.

I have an NTSC 29.97 material that I would like to convert to PAL. The material itself is a mixture of progressive frames at 29.97 fps, telecined frames from 23.976 to 29.97 fps and interlaced frames at 29.97 fps. The problem is with the interlaced segments, which I want to keep interlaced or at least doubled to 50 fps - the interlaced segments can be both top field first encoded and bottom field first encoded... So if I deletecine the material, speed up the remaining frames to 25 fps, all is good, except the segments which were bottom field first - there is the obvious back and forth artifact happening. The segments encoded in top field first are okay.

So, how can I fix this using ffmpeg? Is it possible to do so? I was thinking about something like select='eq(interlace_type,BOTTOMFIRST)' or using idet and metadata:select=key=lavfi.idet.single.current_fram e,value=bff:function=same_str and then fieldorder=tff or phase=t but I can't get it working.

Thank you in advance