Without any nodes above water, that plant will suffocated and die. Reduce the water or lift the plant up so at least 1 node is above the water surface.

If I put cuttings in water it's only until they start to form roots, then they go into a potting mix. The black coloured roots look like they're rotting. What's the water smell like?

Agree check those roots. If the petiole (leaf stem) is too far under water that may also be causing the leaf to yellow rot. Pull it out, trim off any dead/black/mushy roots. Rinse well. I would go ahead and pot it in soil (A nice lightopen/porous mix that drains well but holds some water too....somewhere between an orchid mix and regular potting soil). Else you could put it back in fresh water ultimately it will grow better in soil. You can put pot and plant it in a large open clear plastic bag......leave the top open to keep the humidity up around the plant for a week or two after potting until it gets new roots into the soil.