Are these movies you reported lately all converted to MKV.4K.HDR10.H265 with Video Converter?
If yes, this problem had reported to the relevant department, we'll test on your end.
If not:
Which mode are you using?
Is this issue isolated to this particular video file, or does it occur with other video files as well?
In order to help you better and check where the issue is ASAP, please send us the screen shots in the attachment to list the process you went through step by step, which will be more visual.
Please upload your video source to googledrive or dropbox, and then share the download url with us. Thank you.
Please make sure to update to the latest version and send the Unifab log for check.