Odd it is working for me (version 22).
Yes it is in mm but I don't use points anyway.
If you don't override any of the lines, can you edit in the main 'Marker Text Style' section - this all becomes un-editable when a line override is chosen
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10