Crikey, you know what I did? I lost the spacer (FC-RE105) when taking the rings off, and, like an idiot, remounted them without a spacer. I was wondering about it at the time, but, again, like an idiot, thought maybe this particular one didn't have a spacer for some reason.

So I tightened the bolt going into the crank arm all the way, and did indeed put the small ring out of true, though in the opposite direction of your speculation. That would explain the chain-dropping, which stopped when I loosened that particular bolt to make the ring true again. Boy, I feel like a numpty, but in my defence, I fell out of cycling over the past couple of years and haven't done any wrenching for a long while. I hope to commit many more silly errors as I ramp up my cycling again (spoiler alert: I got a CPAP machine and it's changed my life!).