micheal, I tried to reproduce your issue, though couldn't achieve it at all. You mention "just a simple microflow that returns a list of records" It looks like you use a microflow as a datasource. though you can't aggegate a list with a microflow as a source. A calculated value cannot be aggegated as well. So leaves me with a datagrid which is either populated by the database or an association. both works fine here.
Can you specify which data source & form structure you use?