Credits to @AlfredKamon for discovering this. From some testing that he and I have done in regards to expanding the number of classes you may have, it seems that there may be a "cap" on the maximum amount of standing map sprites. It seems you may only have up to 128 standing map sprites. In other words only indexes 0x00 through 0x7F are valid for standing map sprites. Using a standing map sprite with an index over this value will cause the map sprite to not load/it seems to load a broken version of which ever map sprite would come next if the array looped around. So using index 0x80 for a standing sprite causes a broken version of entry 0x00 to load instead.
Moving map sprites do not have this "cap." This has only been tested in FE8, but I imagine this holds true in all FE games, possibly explaining why FE8 removed so many map sprites from FE7/why some classes share map sprites.
For reference, to my knowledge there are at least 165 standing map sprites publicly available in the community at the moment(this includes custom ones found in the FEU thread). For those of you planning to take advantage of Icecube's class expansion patch, keep this in mind.