What you have there is just a leaf with petiole. What you need is a section of stem which the petiole was attached to. The petiole and leaf come from a node along the main stem of the plant. That is also where roots come from. A leaf and petiole on their own will wither away.

Agree with Jean, looks like he was underwatered for a long time. These guys don't need a ton of water, so don't swing the other way and water too much, but they do need a good drink when the top third of the soil is dry. Conveniently, they will sweat out excess water through their leaves, so you'll know if you need to give it less next time.

I water her once the top two inches of her soil is dry and I keep her in front of an east-facing window. I run the humidifier pretty often as well, keeping the room at a humidity of 60-70%. I also mist throughout the day. She only has one new, healthy growth and the rest are either stunted or growing very slowly and I can’t tell LOL are they not growing because of those dry edges?! :( One of my Calathea’s was just infected with spider mites. I’ve since then isolated her but maybe it wasn’t soon enough, which means... could those issues be from spider mites?! I’ve inspected her soil pretty closely and I’ve only found a few baby gnats (yikes!) which I’ve since then sprayed a bit of neem oil solution on her soil. Haven’t found any sign of webbing either. Please help!! I just want her to get better. Thank you!!

Is that the only leaf that is yellowing? Probably the oldest leaf? Do not worry about one leaf. Are the rest of the leaves less perky than before. Roots grow differently in soil than in water. I prefer to root directly in soil and mixing soil slowly in the water doesn't help the transfer. The plant actually has to grow new roots or at least new root tips into the soil mixture to take up the moisture from it (the soil). That takes a little adjusting and the plant it a little stressed during the process but should be fine. I hope you are using a nice open mix.....one that stays moist but has a lot of air space (e.g. extra perlite and/or bark (you can add orchid mix) in the soil). keep lightly moist. If the plant get real limp you can get a large clear bag and put it around the plant to raise humidity for a few days while new roots get established.....than gradually open the bag over several days. The lighting look probably good..........very bright but no direct southern sun mid-day (mornign or late day sun ok). Never fertilize a newly transplanted or potted plant. Later when it begins growing new leaves you can start some light fertilization but not now. Soil should never be soggy. It should be slightly moist (but have a lot of air space in it, as I mentioned above). Later when it is rooted in you can let it dry between watering.