Thanks, Jon. I actually exported the entire scenario as zip … and did nothing with it.
It seems that the first one is in some weird variant format. I downloaded it from Wikipedia, but /usr/bin/file reports that it’s actually Unicode text. ??? Displays OK in the browser, but I have no idea what’s up.
Since then, I created another very simple SVG (just an arc with an arrow terminating it, to indicate rotation), added that to my scenario (where it was perfectly visible), and then resized it by dragging the top left corner … upon which it became invisible like the other one.
I’ll attach both to this message.