@Zing OK - so after a few days of thinking it was my modifying things or the way I installed things and changing things as a result, it turns out it is a bug in Recalbox's vanilla install. See the video below for an example:
https://youtu.be/encKNBAfjNo
Basically, on a fresh install, C64>Rescuing Orc launches fine. However, if you change Games Settings>Shaders Set to Scanlines, it produces the stretched image error + the game does not load.
Weirdly, if you return the setting to none, the game will not load again. I have also confirmed the same behaviour on the MSX game, Night Knight (whilst not in the video, it runs fine prior to changing the Shader Sets to scanlines).
I haven't tested each and every system, but I'm sure this bug will be there for other systems.
I'll see if there's a bug report system somewhere for rb. 3 days of my life I won't get back on the theory that it couldn't be a fault with recalbox....