Autosave error
When you write an email, the autosave function doesn't work properly. It doesn't save the current state as a draft. You have to wait for it to notify that the draft has been saved to make any changes, otherwise it reloads the previous draft.
For example, if you add a recipient, you have to wait for the draft to be saved (5 seconds by default) before adding another recipient, otherwise it will revert the change to the previously saved draft. The same goes for the subject field.
I had to set the autosave to 0 seconds to disable it.