I too am experiencing this behaviour in 3.5.1 and believe its some sort of bug introduced by ESRI. I have the simplest of models as shown below. On each iteration the field map is lost/reset. The inputs have the same schema they are simply being exported with field lengths being adjusted.

Its really frustrating and makes automation in modelbuilder impossible.

I've tagged a few others who use modelbuilder and may have a solution?

@ShitijMehta , @KoryKramer , @curtvprice