Warzone being a complex game it often brings some new bugs and glitches with every new update. The latest Warzone Season 4 Reloaded brought an old bug in the game that can be traced back about a year. Soon after the update, Raven launched a patch that fixed a variety of bugs in the game including a lot of other important changes.
In the list of bug fixes. Raven listed the gas mask glitch to be the first issue that was fixed. They said they have fixed an issue with the gas mask animation. As the animation was delayed, it caused players to take damage.
That being said, the Warzone gas mask not working bug is very much affecting the game as of writing this post. There are multiple posts on Reddit and Twitter where users claim to have the gas mask on and still died as soon as the gas hit them. Here is a user reporting their problem.
We will keep an eye on the situation and update the post if the issue escalates or if there is an update related to it. The issue was widespread prior to the patch, so it seems the gas mark is fixed for most players.