Arthur,
It's always good pratice to test for empty values, also for decimals.
The error shows that the microflow is being asked to check if null > 1 this will result in an error.
Without addional information on the content of the records that are being processed in your loop it's not possible to say if this is a bug in the modeler/runtime or if this is default behaviour. You could try and check the content of the field of your object with the debugger.