Pretty sure I figured this out.
The counterintuitive thing here is how Guardian Armor works. It's not mere Bullet Resist, it's more akin to how armor tends to work in video games.
Examples:
- Warden with 100dbp (damage per bullet) does ~87 to a Guardian, but Warden with ~20dbp does ~7 to a Guardian, flat decrease of ~13.
- Bebop with ~5.3 per bullet hits a Guardian for ~1.85, but Bebop ~22.2dbp hits a Guardian for ~18.75, flat decrease of ~3.45.
- Grey Talon ~23dbp, hits Guardian for ~8, but Grey Talon with ~46dbp hits a Guardian for ~31, flat decrease of ~15.
Mathematically this is equivalent to taking ~65%
(or possibly 2/3, the game doesn't give you super accurate numbers in the UI) of your starting damage and always subtracting that, no mater how high your damage grows. This obviously means that larger damage > less relative reduction.
So the reason the damage seems odd is because it adds the 35% from Monster Rounds to your initial damage and then subtracts the flat armor. So for example for Grey Talon you end up with 23*1.35=31.05, remove 15 from that, you end up with 16. That's also the reason why in my example with Wraith at max level, the relative increase is far smaller, due to the flat armor having way less impact at the larger damage numbers.
In your example showing the level 1 Bebop with Monster Rounds your damage per bullet is 5.62. Add 35% from Monster Rounds, remove the flat ~3.45 and multiply by 66 bullets: (5.62*1.35-3.45)*66=273, which—barring minor inaccuracies from me being too lazy to use infinitely long decimals—is essentially the damage you did.
So I guess it isn't buggy behaviour, just obscure mechanics we didn't know about.