Multiprocess mode consecutive file issue 3.7.6

[Vextryyn]

using 3.7.6
Issue when using Folder mode or a second run error when starting second file. says that one of the files is in use: The process cannot access the file because it is being used by another process C:/...... split_video0.mkv(also these didnt start as MKV, it always seems to output to MP4 and the files in question are also just so happen to have been MP4)
Workaround: when individual run is complete completely delete working folder and do second file or restart client. unable to workaround Folder mode.

Single process works just fine without error.
would like to use Folder mode in multiprocess as it is significantly faster.