media composer 2020.12 does not export 50i sequence to mov - h264, h265 or mp4 if export is set to 25 frames

iMac pro 10 core 64 ram os Mojave 10.14.6

Do you mean if the project is set to 1080p25 as opposed to 1080i50? I thought this was normal. You cannot create motion FX either on 50i sources in a 25p project. I always change the format temporarily for such clips, give that a try and re-export. 

project is set to 1080/50i export 1080 25p

Then change your project format to 1080p25 and try again. If that doesn't work do a video mixdown in the same format first.

thanks, you can handle it in different ways, I just report the problem.

This is a bug in 2020.12, but not 2020.10