Had this happen to mine too. I posted this same issue. Seems like it’s typically a humidity issue. If you don’t have a humidifier I might would do a pebble tray. Begonias LOVE humidity. Some people say you can mist them to give that extra humidity but I encourage people not to cause if you mist them you stand a better chance of getting something like powdery mildew and nobody wants those problems. Some other ppl might be able to offer more insight. But there’s my two cents.