The Map Room achievement requires that all the maps in a 3 x 3 array of item frames be fully filled out, that is, that there are no missing pixels. This issue is usually the result of one or more maps missing pixels, because this can be very difficult for a player to see:
- For a map being held in the hand, missing pixels have the color of the map background, which is a light tan color similar to sand or sandstone. If it happens that a desert or beach is near the corresponding location in the world, it is very hard to spot the missing pixels.
- For a map mounted in an item frame, missing pixels have the color of the item frame beneath them, which is mostly reddish brown with some light birch-type colors around the edge. If it happens that the corresponding location in the world is in a mesa or contains regions of red sand, various colors of terracotta, a structure made of birch wood, etc. it is very hard to spot the missing pixels.
If you've already checked for missing pixels but overlooked one for reasons such as the above, it's understandable that you might conclude that it must be a bug. But if it's not actually a bug, the devs won't be able to find a bug, and in that case they set the bug report aside to try again later. This can go on for months before they give up, so you could be waiting for all those months only to be told that no bug was found. Then you're right back where you were, looking for missing pixels. The reality is that most people don't seem to be having this problem, so it probably isn't a bug.
So what you need is something to help you find missing pixels more easily. I have attached three resource packs that can help. They change the backgrounds of maps in hand and the backgrounds of item frames to a bright color that you're not likely to find in maps. There are 3 colors available, Cyan, Red, and Yellow. Here's an example of what the yellow one looks like:
They're even more helpful for maps at levels above 0/4. For example, this map is level 4/4. I activated this map in creative and flew around to fill it. You can see where I deliberately circled back just a little too far from my earlier course, leaving 3 yellow pixels that didn't get filled.
You can activate these packs either in a world or in Global Resources.