Burn Barrel Emitted Toxic Gas?

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Buddy and I have a private (password protected) server we play on where only he and I play. We threw items items in a fire barrel and a few minutes later it emitted toxic gas. I went to go close the lid thinking it was normal smoke - wrong. It even had the toxic gas color. We both contracted symptoms of toxic gas and died shortly after. It's vanilla settings, no mods. Barrel was not in a static/dynamic zone. Barrel was placed outside, under the roof in the type of shed pictured. This was on C...