I think your question is missing a word. Is it "Does anyone know if" or "Does anyone know why" ?
Sorry Yes.... WHY have they been painted over?
Originally I thought it was for Adelaide Cup by the gate, but its the entire length. so it looks like bikes are not allowed any more? which makes no sense.
At worst it is still a footpath (unless there is a "No bicycles" sign) so bicycles are still allowed anyway.
My guesses are either (a) it is being repainted, or (b) vandalism.
Suggest your next step is to contact an authority.
It might even be overnight graffiti / vandalism by unknown persons (unless caught on CCTV).
P.S. Mike Tutur Bikeway has the highest count of cyclists for suburban Adelaide.
It's happened before, bike symbols near Goodwood station were maliciously painted over a couple of times. Just report it to the council concerned.
That would be Marion Council then. The tramline is its northern boundary, and the MTB is south of the tramline along that section.
I rode there tonight and it looks like graffiti/vandalism to me. I have emailed Marion Council.