Adobe Acrobat 2017 - Arial Font Merging Issues

Hey everyone,

One of my clients recently has an issue with merging two pdfs that both contain the Arial fonts. I've dug around did a few fixes with Distiler and Adobe PDF printer preferences but nothing seems to resolve the issue.

Here's a picture of the problem ( imgur(dot)com/EzcYcOv.png )

I've also reinstalled the Arial font, but no help.

Thanks in advance!