mkvalidator ERR201 Invalid 'CueRelativePosition' for profile 'matroska v2' in CueTrackPositions
When running mkvalidator on a file generated by mkvmerge 5.9.0 or later you see a lot of these messages:
ERR201: Invalid 'CueRelativePosition' for profile 'matroska v2' in CueTrackPositions at 1616753446
Are those files invalid?
This is a bug in mkvalidator in versions up to and including 0.4.2. It has been fixed in v0.5.0. If you're running an earlier version please upgrade to the latest one.
The in-depth explanation
mkvmerge creates files in which the DocType is set Matroska v4 but the read DocType is set to v2. In layman's terms this means that a player must support at least the elements from Matroska v2, but that elements from Matroska v4 have been used as well. A player supporting v2 but not v4 should still be able to play the file just fine by skipping the elements it doesn't know about.
During validation mkvalidator wrongfully checks against the elements allowed by v2 and not against the ones allowed by v4. It will then encounter a lot of elements belonging to v4 and wrongfully report them as not being allowed.