Couldnt find this solution but figured it out on my own. My "Map Room" achievement wasnt unlocking and it definitely wasnt an unexplored pixel.

It was because we had other maps displayed in our area (treasure maps and other locator maps). All the maps were scattered and in different areas of our town. We ended up taking down all the maps and putting back our 3x3 grid on a wall and it FINALLY unlocked.

So these 9 maps must be the only maps in the chunk or loaded area or whatever in order for the achievement to unlock. We took all the maps down within a 75-block radius just to be sure. Playing on an XBox, 1.19.51, with the PureBDcraft resource packs. 

Hope that helps!