Yes, it is a great tactic to wait until they start spawning.
About the coffins, your friend is right. There are three things that could have happened; 1: there were goblins at the other coffins at the same time and you didn't notice. 2: the other coffins already fell in previous rounds. 3: this was an actual bug. Out of the three the 2nd is the most likely, followed by the 1st.
Thank you for playing!