Scenario: I’m entering text into OO. As I’m typing I may want a word to be bold or italicised, so I Cmd-B or Cmd-I. When I’ve finished typing the words that I want styled that way I deselect the formatting option by toggling Cmd-B or Cmd-I. However, the words after what I’ve typed now have a different style from the words before the bolded/italicised words.
This is a bug, we’re not keeping track of enough information to handle this properly in all cases. What’s going on is that when you use cmd-b to unbold, it applies a new font weight of “regular” to replace the bold weight, were regular the default, that would simply be removed and all would be normal. But in your case the default weight is medium – pushing cmd-b applies “heavy”, and turning that off with cmd-b is applying “regular” which is less than medium. In this particular case the logical thing to do is simply remove the heavy attribute so it goes back to the default of medium. But if the default is bold or heavier, it needs to be smarter than that and apply the appropriate weights.