webm recording not always correctly converted to mp4

webm recording not always correctly converted to mp4

The following error and warning being reported to console:

More than 1000 frames duplicated
Too many packets buffered for output stream 0:0.

According to https://superuser.com/questions/1308355/ffmpeg-webm-to-mp4-awful-video-quality

adding

-vsync 0 after -crf XXX

might help