MediaCoder

Looks like new Opus encoder has no CBR mode. Avoid using that.

Have you tried latest version?

Which version are you using?

Could you call out the console window (press F6) and give me a shot?

It just has improved things you don't mind.
Have you ever tried popping in start/end time?

Looks like a bug. Will fix it.

Support of VCE in FFmpeg is too limited 3 years ago. It's improved after FFmpeg 4.0 is released. I will work on this again.

Width and height are rounded to 8 dividable.

Set video source to FFmpeg.

You can just download K-lite codec pack (or anything similar) instead.