I find it odd that the level can be launched. There should be an error message. I just remembered something. If you cheat-save in KS+, then load the level in KS, you'll be in the wrong spot. Normal saves point to a specific tile, but cheat-saves point to a specific pixel. So, if you cheat-save at pixel x500y200, original KS will start you 500 tiles to the left and 200 tiles down. That would put you in the wrong room altogether, most likely a void.
So if you're cheating in KS+ and want to switch to original KS to finish a level, be sure to save with a legitimate save spot first.