This is not a side effect that I've heard from Propanolol so far. Could it be something else? I know my kids would both, when they were babies and even now, go in cycles where they would eat more or eat less than normal. They still do it AND they always stay on the same cycle. AND their sleep cycles coordinate too. When they are eating more, they sleep a lot.
I always know when they start eating heartily again, I'm going to have to go clothes shopping.
Keep monitoring it though and keep the pediatrician informed. I would think if it drops any lower, it might be a concern.
Let us know if this continues, too. I'd love to know if you do think it's a side effect or just natural.