I can't say we've seen this bug before; what operating system are you running on, and are you using integrated graphics or a dedicated graphics card?
Additionally, what version of DirectX do you have installed?
Win 10 Pro 64x v1909 18363.778 build, dedicated (but hella old) NVidia GeForce 9500 GT, and DirectX 12 according to dxdiag. The map's background graphics came back before shutting the game down in the same runtime (i.e.- without closing and reopening the game) after I'd beaten the game and decided to Continue and start a New Game, though.
If it helps, I first noticed it had disappeared around the time I reached the ???? platforming puzzle room with the fragments and thought it was deliberate, but then it remained gone for the rest of the game after I had left said room (you can see I'm in the hub, left of the boss door, in the screenshot).
I should probably note I'd been going through the paths in reverse order, just for kicks (i.e.- started with the P6, went counter clockwise). So I'd been playing for a while before I reached ????. I also revisited each path once because I was looking for secrets and checking if the achievement list in 0.27P actually functioned.
Edit: Unrelated to any bugs, is it supposed to be possible to obtain the items marked with question marks that lack the "This item is unavailable in this version of the game" in 0.27P? Are they locked to the demo's Easy mode, if so? (They're the Four-Leaf Clover, Marble Chisel & Hammer, Rugby Helmet, Pair of Dice, and Blender, based on their presence in 0.45a.)
Both of the rooms other than the MainFrame's in both of the 3x3 grids' bottom row are labeled with the number 8 on their background, so there's no 7 or 9. Is this intentional so as to help hide the boss room's position in each?