an empty <p></p> will use the settings of textformat default
your textformat default does not have space top or space bottom
please use textformat's space top and space bottom settings to create spaces
you could also style empty p's like:
p:empty{ margin-bottom: 40px; }
anyway, i wrote down this problem and will take a look, but not sure when i'll find the time
also it doesnt work for me even on reload