Consider the following ZDoom screenshots:
Those were taken across a few different boss deaths. It's not a strong bias, but it is definitely there and very repeatable.
As I mentioned, it's not just ZDoom.
However, EDGE seems to have a totally different algorithm for spawning the explosions.
So, anyway, I assume (always a dangerous thing to do) that the explosions are meant to be properly random and spread reasonably evenly across the icon's face. I know that random is often not as random as you might expect where computers are concerned but I wondered if there a reason in the code that anyone has spotted for this slight bias?