Gas damage used to be multiplied vastly more than it should by stealth.

Like a normal hit of 100 damage becomes 800 damage on stealth. Assuming a normal Gas proc dealing 100 Gas damage causes a Toxin AoE dealing 100 Toxin damage and a Toxin DoT dealing 100 damage; A Gas status effect against stealth would deal a Gas hit of 800 damage, and Toxin AoE of 6400 damage, and a Toxin DoT of 51 200 damage.

It's not supposed to do that any more, but it's possible it bugged again at some point, especially in the Simulacrum, and even more so in a multiplayer Simulacrum - that game mode has been plagued with bugs that have not appeared anywhere else in the game since it was introduced.

It is worth noting that the Bane mods (something like +Damage vs Corrupted) still multiply Gas status effects like that, so if you had any of them, that could be the answer, or they could have triggered the stealth mutliplier bug.