The bit I misremember is GL explosions piercing bullets dealing half of what bullets explosions deal(not the same, don't say bullets/explosions deal X, this part stays true after the correction) and being bugged to not benefit from some things(damage per soul heart being one of them) because they're hostile.

I don't know the source for that claim but GL explosions clearly are not halved. You can start a run and immediately your explosions are doing 60 damage, which is GL's base damage.

Maybe as you say there's weirdness with the scaling (either a bug or just hidden formulas that are more complex than described).

I just tried another run in which to test the Soul Heart claim, and it turns out I think the Tome of Might (+2 damage per max hp) is also bugged for the GL. I did D0 with no runes and just built for summons without taking bullet damage anywhere else, and with 5 max HP my grenades went from 60 to 50 damage! Conversely, though it is much harder to tell, I think it did increase damage of my scythe, spears, and glare. Glare for sure was doing 25 after taking the tome, and after getting another +1 max HP, glare was 27 and GL explosions 48.

Continuing on some more I got the +damage per soul heart and you were right, it does not seem to scale the explosion at all. Also checked to make sure that was still the case after getting Blast Shield (GL upgrade to be immune to explosions).

So I guess avoid the Tome of Might with the GL, that and SH not scaling GL was why I was doing less damage after so many upgrades. I wonder if Death Rounds also reduce its damage but I didn't want to do another run to build for it to check.

The source is my dumb ass getting basic ♥♥♥♥ mixed up. Thanks for correcting. I mixed up which is halved for piercing purposes. PIERCING BULLETS deal half so the max of pierce + explosion is 1.5x of just the explosion.

You could be seeing a bullet impact, but now I'd rather not be sure of anything.

Also, I got tome of might and restoring health did increase explosion damage but not impact wtf?

I'm just going to chalk it up to weird bugs. I also noticed one with Witch, her crow upgrade (+1 crow and crows deal bullet damage on contact) and some other upgrade that caused their damage to be reduced... but only until I paused the game and tabbed out, then tabbed back. Or, I mean I'm hardly doing scientific testing so maybe I'm just looking at the wrong numbers.

I finished the achievements so I'm just going to shelve the game and not worry about broken mechanics in edge cases.