Probably an old leaf and ready to fall off (one of these weeks). Keep it moist but not wet and new leaves will appear when it is ready. My rotundifolia was nice indoors but every single leaf dried up when I stuck it outside for summer and neglected it when I had some travel. Since that time it has come back nicely.

Jan 29 in the basement between light stands.

I don't think I have any photos of it when it looked dead in early summer but it was so ugly I cut the dried up brown leaves off. It took a few weeks for new leaves to start showing up.

Here it was Aug 19 with all new leaves.

Calathea are notoriously picky/sensitive about water. At best mine get rain water or at least filtered water. Are you giving it tap water?

Sometimes when standing near a humidifier the air coming out feels cool - that could also cause this. Why do you say it's not browning around the edges? It is.

Also, you say you water when it's moist, almost dry. Which one is it? Moist, or almost dry?

I am watering it with filtered water. Humidifier is also using filtered water.

I’m keeping it at a distance from the humidifier so that the mist naturally falls onto the plants versus it blowing directly on it.

I water it when it is still moist, to the point where when I stick my finger in, some soil sticks to my finger but not a lot.