Is there a max number of data sources that can be used with an aggregatedatasource?

I have an aggregate data source (sharepoint:aggregatedatasource) used by a dataformwebpart that is functional in 2010 but not in 2013. In both environments, the aggregate data source has 18 spdatasouce nodes in its sources property. In 2013, only data from the first ten spdatasources shows up. Is ten a hardcoded limit now in 2013? Is the fact that it's cutting off at ten a red herring and there's something else wrong?