Loose material rattling inside bronze statue, is it common?

Depends a bit how it was made, but while it's not indicative of very careful production it could be leftovers of the material the bronze was cast into. If it was cast hollow in one piece it can be hard to get all that material out. Though really it is not entirely professional to leave some in, as over time it can lead to some corrosion. Though not in the next decade or two. But depending on price and way of production it can happen.