If that file is from this batch - it’s broken:
Specifically if the file is from the above batch with internal Created Date on or before 5/4/2020. The original file can be fixed by simply remuxing with MKVToolNix Version 45.0 or newer.
If the file isn’t in it’s original state - it can’t be fixed.
Reacquire - then fix.
Careful reading of the linked thread will reveal if your file is one of ‘The’ Files.
