Kubakiri still executing rulers

[Thisnameismeta]

I fought the viper king in its nest and managed to execute the viper king using a sniper with Kubakiri. No dmg pop up appeared, just the stock executed message from vanilla's repeater. I take it this is not expected behavior judging by the perk's description? I'm using a few QOL mods (free camera control, gotcha, overwatch/evac all) and a couple mods to modify sparks, but I can't imagine that would affect this mechanic. I can provide a save game and more detailed mod list if desired.

[Franzy]

I confirm it. Executed Berserker Queen the other day wuth Kubikiri. She had 53 hp left, while maximum possible crit from my sniper was 24.

[Franzy]

Update: Just met Archon King, kubikiri DID NOT kill it outright, but dealt 56 critical damage, which seems quite right 200% critical damage (plasma lance).