I have a support case open. Yesterday, MS Support acknowledged the issue, and said they are targeting the end of September for a fix. They 'archived' the case for the time being.
I provided Support a few PDF's that exhibit the issue. It is only some PDF's. These PDF's are vendor invoices sent to us by various companies. We are seeing the issue invoices from specific vendors. I cannot tell what authoring tool was used to generate the offending PDF's. Regardless, the PDF's work fine outside of SharePoint preview.
So we'll have to wait for MS to fix the issue.