I've tested the scenario and I'm able to notice the same problem. I've logged it as PDFNET-13097 in our issue tracking system. We're looking into the details of this problem and will keep you updated with the status of correction, within this same forum thread.
After initially studying the issue, we’ve concluded that when a paragraph is set as PositioningType.PageRelative it’s treated as a floating paragraph, and unfortunately, IsFirstParagraph no longer affects floating paragraphs.
As a workaround, please avoid using the PageRelative positioning type or use PositioningType.Auto but be aware that it might not fully resolve your problem. In a simple test on my end, it worked correctly.