Yes, I confirm that this is the solution!
Try like this:
1. Play 2 (or 3) games until the finish
2. Quit to main menu
3. Repeat steps 1. and 2. five times
You will (I hope) then get the progress 5/5 and complete the challenge "Fastest hunk of junk".
I have tried this at least 10 times and it still wont update the challenge for me, I am still stuck at 4 out of 5.
I also occasionally see the little 'Milestone Achieved' icon pop-up for the Heap of Junk challenge at the end of certain matches but when I return to the main menu it still says I have one left to do.
