I have tried your character "PowerOutageTooStrong". I used normal rarity Dunes maps with no mods to test. I did not use any flasks (they were all purely defensive).
But I did not see any appreciable difference with/without Belly of the Beast. Other than the Extra Gore, of course. I could still explode entire packs either way, most of the time. However I noticed a few things that might be causing confusion...?
Your main hand chance to bleed is 95% (as seen on your character sheet) so you occasionally fail to apply bleeds to some monsters.
You are using the Vulnerability aura, but the radius of the Aura is smaller than the distance of your Lacerates, so you are sometimes getting the benefit of it, sometimes not, depending on your distance from the monsters.
Auras don't update every frame, so if you are moving and attacking fast enough, it is possible to hit nearby monsters before the Aura has updated to affect them.
You have two Lacerate skill gems equipped. I used the one in your main hand (supported by Chance to Bleed) and removed the other one, but there is potential for confusion here.
You have Herald of Purity active, and the minions will sometimes wound or kill monsters before you get a chance to apply bleed to them (also, nearby ones are buffed by Dread Banner, for additional damage/randomness).
You were in Sand stance, rather than Blood stance, at the time of testing. Not sure if relevant, just mentioning for clarity.
You have a Tabula Rasa corrupted for extra damage sitting in that character's inventory. If your "non-Belly" testing used this, then that would've affecting your damage also.
You have Ryslatha's Coil equipped, which increases the random range of your physical damage (which is most of your damage).
To clarify, you won't trigger bleed explosions if:
You miss the bleed proc with your main-hand (on enemies outside of Vulnerability) then kill them with the offhand hit.
Your minions kill enemies before you can apply any bleed.
You "one-shot" enemies already wounded by your minions.
You legit one-shot enemies.
Note that "one-shotting" can be a bit random due to damage rolls, but especially so with Ryslatha's Coil.
Additionally, it is worth mentioning that pack clustering is another important factor. A densely clustered monster pack will "pop" much more easily due to overlapping bleed explosions finishing off monsters that were bleeding but not killed from the initial hit(s).
Overall I didn't have much trouble blowing up monster packs with your build, with or without Belly of the Beast equipped. It wasn't always 100% consistent but generally seemed to work well.
Let me know if I've missed anything. E.g. I'm not sure what maps you were running and I didn't really use your Leap Slam or Double Strike much, just ran around using Lacerate in various ways... But basically I could not see any bug here.