Colin, I'm facing the same sort of problem.
I would like to map an Excel column to an integer attribute and use a parser MF to limit the allowed values. E.g. if the value is >5 then make it 5 etc.
When mapping a column to an attribute of type Enum, DateTime, String or Boolean my parser MicroFlow is triggered. When the attribute is of type Integer or Decimal, the parser MicroFlow is not triggered for some reason.
I have not been able to debug the issue in Java, because most of the mapping/parsing is done by the classes in replication.jar file, where no source code seems to be available from.
I would suggest reporting this issue to Mendix as I have no idea why a parser MF is not triggered when mapping to an Integer or Decimal attribute.