I made a post about this guy a while back about it being too dry then too wet. I ended up reducing the watering to let it dry out a bit. It would push out a new leaf but it would fall off almost right away. I noticed that the stem died in the middle so I cut it and each half of the plant only had one leaf. I put the top in a small plastic cup with some sphagnum moss and it has now grown small roots and is pushing out a new leaf! The half with the established roots was reported in a larger pot and has made 2 new leaves and is pushing out another one. It seems to be doing much better and is much happier!
Here is the old post.
https://www.reddit.com/r/begonias/s/ge2jof6EUj
by DrBubbles42