I've kind of had this issue for the last few versions of ArcGIS Pro. When trying to perform an operation such as Append or Merge that uses the field mapper, the field mapper doesn't retain the modifications that I specified. For example, if I change a field from "Prefix" to "ROADPREDIR", when I click away the mapper reverts to "Prefix".
To work around this, I've had to use the Alter Field tool for each field that I'd like to change when migrating data. I'm not sure if this is a bug or what. Just putting this out there.