This issue was already reported by Ken Lesniak on 08/2019.
Core mishandles date tags in yyyy-mm format
At least on Windows, the problem still exists.
Core: Windows Server 2016
ROON 1.7 (build 710)
If you specify a date file tag (for example YEAR or ORIGINALRELEASEDATE) and format it as yyyy-mm, ROON adds 01 for the day when it’s imported
This formats work as expected
yyyy
yyyy/mm
yyyy-mm-dd
yyyy/mm/dd