D&D 5E - Cloudkill when you play it RAW
But here's where we part ways. I don't argue with the DM. I may disagree with them and discuss it after the game, but argue? Nope. During the game my opinion on how I would rule doesn't really matter. If I disagree often enough, I'll find a different DM.
To each their own. Around the table, we're all people trying to have fun playing a game. If someone isn't following a rule, I'll bring it up to them, regardless if it's the DM or player.
If I leveled up, picked my spell assuming that it works as written in the book, gave a copy of my character to my DM like I always do so they know I now have that spell, and then mid session I got surprised with "oh, I'm going to make it do something explicitly different then the book says", I'll argue.
Mind you, that's different then a DM making a ruling or clarifying something unclear. And plenty of times I question something the answer is "there's something you don't know so it's working like X" - that's more than acceptable.
But it's unlikely either of those hold in a case like this. Now, what I may arguing for may just be to "let it does as I expected when I cast it (i.e. what the book) says for now, since that was my expectation when I cast it, and we'll do it your way moving forward".
This case is a fairly small change, I'd mention to the DM to see if they just misunderstood the rules but only argue if it invalidates my character's plan in a big way.
And again, all of this is for a surprise, mid-session issue. If I knew about it previously, I've either accepted it outright or brought up my concerns to the DM and we've gotten to the point where I accept it.
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