Important info regarding the Firemace:
Heretic's Firemace only has a "~75%" chance of spawning in total at one of several Firemace spawn points placed on the map. Even then you can only place a maximum of 7 spots for the mace to spawn. When the MaceSpotCount reaches 8 the game has an error. In deathmatch, the firemace will always appear, but there will still be only one on the map.
So the Firemace weapon pick up doesn't work the same as the others as it's not guaranteed to spawn in, even if you only have 1 Firemace spawn. It's coded this way to be a rare weapon as with the Tome of Power it kills all non-boss enemies in one shot being extremely powerful.
Thank you to @SilverMiner, @lobo, and @zedonk for the information.
Testing a bit I'm not sure just how random it is which mace will spawn out of several 😵