This is not reproducible in ArcGIS Pro 2.7 Beta.
The first append using the feature layers and the Maintain Attachments environment setting enabled accomplishes the transfer of the records with photo attachments to the target dataset.
The issue is tested in ArcGIS Survey123 feature services. Appending two records each with a different photo attachment into a second ArcGIS Survey123 feature service (matching schema) resulted in the two records transfer to the second feature service, and the attachments are maintained.