Sorry about not responding yet. I couldn't find anything about it.
Also, If you want the loot, DONT. BREAK. THE. CHESTS/POTS.
I don't now what happened. I will use the modpack you have and try to see what is causing this problem.
It is a glitch. It happens to me too. The loot just hasn't generated yet. Wait a bit, then you check. (Don't do what I said if your in a dangerous place)
PM is a very good boss, and is supposed to be the last boss you fight of this mod. (Besides the Prime Wight) That is likely why he is the Hardest boss so far.
I like PM and DPM both, But I voted for DPM