Matroska Tags defined as numerical should be.
Some tags are defined to be a number in a UTF-8 element test that the value is numeric.
ffmpeg -f lavfi -i testsrc2=s=120x80 -f lavfi -i sine -c:a flac -ar 8000 -vframes 2 -c:v ffv1 -level 3 -c:a flac -g 1 -metadata TOTAL_PARTS="Many parts" -y MKV-NUMERICAL-TAGS.mkv