I have always had the same issue, especially, obviously, when I have scanned old photos.
Unless I missed a response with this information, it would be so much easier if Fstop would just tell us how they prioritize what metadata fields they use to sort by “date taken”; EXIF (dateTime), IPTC (Date Created), XMP (CreateDate). Then if there is no data in these, what is their next choice.
And in sort by “Date Taken, modified”, which field do they use for date modified, and what is their priority for these two fields.
That would allow us to change what needs to be changed and not waste our time trying to hack which field does what.

My FStop Pro app version is 5.3.27, and my Device is a Samsung Galaxy S20 5G.