I get the same thing on other types of begonias as well. I always find this is prevalent when growing indoors over winter but once outdoors and acclimated these kinds of problems usually disappear altogether. That would suggest a humidity problem, watering issue, perhaps a cold or hot draft on the leaf.

I know this does not answer the question but I would not worry about this too much. I would continue removing bad leaves or trimming away the damage on the leaf.

Thank you so much! This did answer my question, I was afraid it's going to die on me! And it's a very treasured plant to say the least!