Thanks for that.
I checked my version and build. Was 12.0 5808 Updated to 12.0 5861 and it behaves like you describe. Initially, px for Font size. Enter a value, then you can change to any of the font size "varieties". Maybe I hadn't tried that before or the update has enabled me to do so. In any case, I am happy with the way it is operating now.
best wishes
Peter