Indeed, I experience the same behavior. That's weird. I guess this depends on when the pdf reader / firmware calculates the actual borders. I noticed that hide margin does not work perfect on all pdfs, as sometimes even small parts of the text are cutoff. As a workaround I then set the zoom by myself (selection or 2 point zoom, or even just 150%/175%).
Wonder what might cause this erratic behavior. Next time I find a pdf for which hide margin does cutoff parts, I'll write a bug track for it (see
https://wiki.mobileread.com/wiki/Boox_M92_wishlist ).