I'd rather not use setBackgroundDrawingCallback (doesn't seem necessary) it was just something I tried. So I've removed it now to keep troubleshooting.
I notice I only get trails if I change the sprite's image--as in the text on the right here:
And the trail only lasts ONE frame, not forever.
What the heck could cause these symptoms??
I am stumped! I've ported my background graphics and sprite code over to your example (many thanks!) as best I can and I do NOT see the problem there.
Structurally my app is complex (re-assigning paydate.update, running things on timers) but graphically it's not. I'm just drawing some circles, with a bit of text overlaid as sprites.