I am experiencing an issue with ArcGIS Pro and it doesn't seem specific to a version as I have reproduced it with both ArcGIS Pro 2.5 and 2.7.
When creating maps with an aerial photo backdrop (not an Esri basemap in this case) then exporting to PDF the result looks great and comparable to one produced in ArcMap 10.4.1. However when I print both maps the one created in ArcGIS Pro displays an solid rectangular black box representing the image envelope that becomes visible in print (see attached). I tried a number of things to try and fix this, including a quick test with a simple image in map frame and switching the raster image from a 10.4.1 sde geodatabase to a .sid image to see if this might be due to Pro having trouble reading from our admittedly older geodatbase. The .sid gives a slightly different result (hollow black envelop box) and it is also somewhat visible in the PDF too as I have pointed out in the attached images.
I should mention I have found that if I check the output as image button in the Export dialog this seems to solve it and the envelop goes away which is great, but this is only a workaround as there may be cases where we need to keep the PDF layers intact, text searchable etc. Also it seems there are a lot of people commenting on another thread related to transparent images producing lines in the PDF which is not the issue I am having for some reason. I get the same result whether or not my aerial image is set to transparent.
My current export settings are below, but I have tried a lot of different combinations and I was curious if anyone else is having these issues and/or knows how to fix this.
File type: PDF
Compression: JPG
Quality: Default 80%
Compress Vector Graphics: checked
Resolution: 300dpi
Raster Resample: 1:3 (Normal)
Layers and attributes: PDF Layers Only
Color Management: Embed color profile checked