oosyrag, thank you!
The true solution would be to use sin and cos to determine where the object should be, in effect recreating the orbit with events rather than using behavior. Then you have control of when the position gets updated.
I tried this and was still getting the effect, but then you said
Alternatively, have you tried Wait 0 for updating the helper sprite's position?
And it worked. Haven't bothered to try it with the orbit behaviour, as I now essentially have what I need. Solution below if anyone needs it for future reference. Also, thanks dop2000 and — I honestly hated the idea of pinning the player to the platform though, so am glad it won't be necessary :P
edit - not sure why I can't tag Nepeo, but thanks anyway mate :)