Simply set down a campfire and watch the smoke rise up. You should start to see glitchy lines quickly appearing and disappearing from the campfire smoke. It is very fleeting and actually next to the smoke. This also happens with hay under the campfire, so I think it is just a bug with whatever sprite or particle was made for the campfire. (but what do I know)

This happens in any biome, this is also on realms so the iMac OS shouldn't be the issue. (I would think)

p.s. I am sorry I didn't post a screenshot with this but there are two reasons for it; one is that it is very hard to get a screenshot of something fading so quickly in and out of the game, and two is that even when I did I could not find the screenshot in my files. You should be able to see it for yourself once you set down a campfire though.