Update (I know, it hasn’t been long).
I’ve discovered that if I manually change the source on the TV to SCART, then switch on the Mega Drive, the picture freaks out (this is not normal behaviour and didn’t happen before the mod).
But, if I leave the TV on an HDMI source and switch the Mega Drive on, and let the TV change to SCART by itself, the picture behaves normally.
I still have no idea what’s going on, or how to fix it.
Is the TV trying to change channel… voltage drop… or voltage fluctuation on the aspect ratio select line??