If I want to add a space after mf, I can do that in the properties panel, but not when I enter the dynamic. Is this by design, or should it be possible? I also tried with non breaking space and got the same result.
On Windows using a keyboard with a numeric keypad, you can enter a non-breaking space in the dynamics popover by turning on Num Lock, holding down Alt, typing 0160 on the numeric keypad, and releasing Alt.
It works differently in this case. To obtain more space between mf and the suffix with the dynamics popover, you can type mf, a space, alt+0160 and the suffix.