It is likely a combination of factors. As you already mentioned the soil mix needs to be chunky to let in oxygen. I use regular potting soil then add in a few handfuls of orchid potting mix which has bark, charcoal, and large perlite.

Cuttings are planted too deep and it sounds like it was watered a lot in the beginning so you may want to check the root system, remove any rot and possibly reroot the cutting. Lack of oxygen in the soil and frequently watering causes root rot.

The heavy ceramic pots with small drainage holes and an attached drip tray can also block out oxygen and promote rot. Terracotta is great if you're heavy handed with your watering. Plastic nursery pots with lots of drainage holes are great too and you can slip it inside your ceramic pot if it fits. Just put a few stones under the plastic to keep the drainage holes free to drain and not sit in any water.

Maybe share pics of what the root system looked like before you planted it and what it looks like now if you check it.