Steps to reproduce:
There are cases when notifications can be missed if trigger is running in multiple event generation mode. That happens with single browser tab. To reproduce it, configure a trigger with condition something like "x > 3", set it to generate multiple events, then move trigger to problem state and incrementally continue to send values that fulfills problem condition. You will observe that part of newly generated problems will be missed, but some of them still be displayed.
I'm not sure if this is expected. I just want to pay attention to behavior that looked strange for me. That works equally in 4.0.