I have tested the game on all the emulators that I know of and the error you mention could only be reproduced on old or inaccurate emulators.
This is produced by a bad overlap in the cut area between the two parts that make up the title screen, but I must also mention that this problem is found in the same way in the original version of the game, only that it is concealed by filling in said sector of a solid color.
Personally, it seems like a very vulgar solution, so I prefer to wait and see if that happens on real hardware
On the other hand I just finished a simplified version of the "Zone Selector", although I am not convinced of the key layout, it is quite functional And since I never lose hope I hid enough space to write "Scrap Brain".
What I couldn't help but notice is the horrible color transition that the game has when switching screens:
At first I thought that it would be enough to modify the palette so that the white color was located in another position, but when doing so, the same thing continued to happen with white, I imagine it must be a residue of a discarded feature, perhaps they would have planned to reuse this screen in some way in the level selector, since their data continues to be loaded when entering this secret menu, which does not happen in the rest of the transitions with this itself.
Anyway, I would like to find a way to disable that function, since it is not aesthetic at all, so if someone has the knowledge to give me a hand I would be eternally grateful
They told me that Sonic's skin tone was not correct, so I made the following modification, but since I can't distinguish some colors, I don't know if it's right, what do you think?