This flow step should work for integer fields. You can open up a Support ticket to have this investigated further. Thanks!
I just went through a similar exercise, trying to move data from a custom field into a standard SFDC field and hit a wall. I realized that my custom field is of type "string" while the standard SFDC field is of type "integer."
Is it possible to update the field type of my custom field to "integer" while preserving the data that's already in the existing field? Is there a workaround to this?