In Mendix you two datatypes for numbers: 'Interger' and 'Long'. An Integer has a maximum value that can be set and that is 2.147.483.648. A Long has a max value of 9.223.372.036.854.775.807.
I don't know exactly what you are trying to do in your microflow, but I suggest you have to look over the sums aggregation. Check what if you do a sum aggregation that is larger that the Integer max value. If it breaks, it means that Mendix is aggregating based on an Integer instead of a Long. The best way is to specify a ticket in support.mendix.com so they can have a look at it.