Source Fields list intermittently blank in Append field map (Pro 3.0)

[DataOfficer]

I am using the Append tool in ArcGIS Pro 3.0.1, and am experiencing an intermittent issue when using the field map to reconcile field differences. The issue is that when selecting a field from the left hand Output Fields menu, and then selecting Add New Source from the right hand Source menu, the drop down Add New Source Fields list is sometimes blank. See the image below for an example of the blank Fields list. Other times, it is correctly populated with the Fields from the Input Dataset. I am not changing anything between the Fields list being visible or not.