Guess how Pro is exporting this series. Individual PDFs? Nope.

I look away for a minute and then wanted to open 'one' of the pdfs to see what it looks like - because I don't trust Pro to not find a way to screw up a map. Couldn't find the drill hole I was looking for so first thought I must've forgotten to double check the export folder.
Look again and recognise the project name sans suffix. #$#$!
Check the export settings and they are as above.
Worst still, there is a UI bug that means the single page option isn't even listed after you cancel the export.

You may be able to fix it by clicking around to another map or window, selecting the layout again, going to export again, and maybe it will show the correct options; maybe you have to restart Pro.
Maybe it's fixed in 3.4 but that apparently has PDF export issues.