This is the way of it, I'm afraid. We weighed the clarity of swapping out the button help with whatever the user had mapped versus the enormous amount of video memory required to load up every possible piece of button art every time you entered the Inventory, and speed/memory won out.
That is as planned.
Listen, this doesn't make any sense as an explanation. Aside from the fact that it couldn't be more than a few kilobytes of memory because it's just displaying text on top of a square, and even if it were being done such that the art was huge, they would only need to load the ones being used.
In the key binding menu they're able to "load up every possible piece of button art" just fine. That's where you re-map the keys in the first place, and all of those display correctly when you're remapping them. So seeing as they already have a menu in which this works, and it doesn't eat up enormous amounts of video memory, this can't be the issue.
I suspect the real reason that the button displays don't update is because the programmers found that the keys didn't remap properly, and so even when they changed the menus to match the remapped keys, they found it didn't display the correct controls. Rather than address the root of the problem they just left the menu displays static, so that players who remap their own keys are basically aware that they're doing so at their own risk and can't expect the menus to work after that.