I can't get the equation of a karnaugh map
Yes  $$MD+F\mathop{\overline{M}}$$ is the right answer.
The upper vertical group 11 is for M=0 and F=1. You do not consider D since it is the variable that is changing state inside the group.
Check this: https://electronics.stackexchange.com/a/555144/254042
You do not need to rethink the Boolean simplification properties when using K-maps. You apply rules and get the simplified expression otherwise it would be useless.