If stuff like this happens, there are two potential reasons: A bug in Acrobat (which is rare, but happens), and a problem with the PDF file (much more common). You said that this is a scanned PDF file. Was the file generated by a scanner, or scanner software directly, without support from Acrobat? If so, I would tend to blame the PDF file.
There are several things I would try: Do a Save As in Acrobat. and then open the new file. Does that behave differently? Run the "Auto Tag" function in Acrobat. That has corrected some problems for me in the past, but I doubt that it would do much in this case. Try to export just the image from Acrobat and import again into Acrobat and then export the resulting page. Another option would be to save this one page as image (this is different from exporting the images) with a lossless compression method and a high enough resolution (e.g. TIFF at 600dpi) and then import this image again into Acrobat to create a new PDF file.