Players who like many others absolutely hate the Ultimate and Tactical abilities of Caustics' poison Gas Grenades and Traps were flocking to Pathfinder and Revenant due to their assumed passive ability to take reduced damage as synthetic individuals. Apparently to Apex Legends developers Respawn Entertainment, you were quite wrong.
Respawn has confirmed on their Trello board that this behavior is indeed a glitch/exploit which is in turn sure to make anti-Caustic fans upset. Nothing can be more frustrating as being locked down by Caustic as his Gas Grenades to significant damage to you while he suffers little to no consequence.
Respawn Entertainment to Fix Pathfinder & Revenant Damage Exploit
The developers have labeled this open ticket as - in list investigating - and we can expect a full hotfix to happen within the next few days, depending on how long it takes to sort out the issue. It's worth noting that the ticket mentions "Both Pathfinder and Revy take less damage when in Caustic's Nox gas," suggesting that this affects not only his Ultimate but his Tactical ability as well.
Other issues that have crept up in Season 5 include a glitch with Loba's Tactical and Wraith's portal. Although Respawn addressed Loba's teleport glitch, it apparently seems to still be happening for select users. With so many unintended bugs in season 5, the devs are no doubt hard at work to keep the content coming while staying on top of any bugs that need to be addressed.