There aren't many reports of this bug, so it seems possible that lots of players are successfully getting the Map Room achievement. It's even possible that this bug has already been fixed, and you're not getting the achievement for a different reason. Depending on how you attempted it, making your maps may have taken a lot of time and effort, and trying to fill in missed spots even more. Maybe you became frustrated, bored, and convinced that there was a bug in the achievement. So you came here and found this report, and now you have an explanation that satisfies you.
But you might be cheating yourself. You might still be missing a pixel somewhere that's blocking you, and you don't know how to find it. If you stopped trying because of this report, when in reality there's no bug or you're missing a pixel, you'll be waiting forever. That would be a pity, so I wanted to suggest some things you might want to try before deciding this bug is causing your problem.
For completeness, first I'll describe the requirements to get this achievement. You need to place 9 maps in item frames arranged in a 3 x 3 square. The maps must have the same zoom level, must be complete (no missing pixels), must map adjacent parts of the world, and must be arranged in the order they appear in the world. I'm not sure they have to be mounted with north at the top, but that's how they're oriented upon mounting and there's no reason to rotate them, so don't.
It's easy to verify that you've fulfilled most of those requirements, but the completeness requirement is harder. If a map is missing a pixel, that pixel is transparent, so what you see there is the background behind it. For a map you're holding in your hand, the background is the map paper. For a mounted map, the background is the item frame: the birch-colored frame if it's near the edge, or the leather if it's interior. To detect a missing pixel, you need to recognize that its apparent color is one of these instead of the color of the terrain at that point in the world.
The problem is that the map paper color and the birch frame color are fairly similar to the terrain color for sand and sandstone. (Also, the leather color is similar to mesa terrain colors, although holding it in your hand should reveal this because the map paper color is contrasting.) So it's quite common to miss mapping a pixel in a desert, a beach, or for sand beside a river or pool, and it's hard to detect it. I miss them all the time myself.
If you're not sure whether a pixel is missing, there are a couple ways to test it. You can try to visually compare its color with a blank map or empty item frame, because the latter colors don't typically match precisely the colors drawn on a map. A better way would to make a copy of the map and mount one in an item frame. You can then compare it with the handheld copy looking for corresponding pixels with different colors (map paper and birch wood have different colors).
Tip: Use Level 0/4 maps to earn the Map Room achievement. Higher level maps scale larger features down to single pixels and hide small features entirely, so their images tend to be more pixelated. This makes it harder to notice when a pixel is missing, because there's more contrast between adjacent pixels generally. It also takes a lot more effort to make high-level maps, so by the time you're trying to put them together on a wall, you're psychologically more disposed to decide that colors that don't quite look right are good enough, so you overlook missing pixels. I spent a lot of time trying to earn the Map Room achievement using level 3/4 maps. When I finally decided to try it with level 0/4 maps, I earned it in a few minutes.