I had exactly the same issue with the middle bar when lowering my roof. This is how I solved it:
To buy:
Hardware store: 1m elastic cord 5/6mm color black if available. Costing me 80 euro cents.
Tools:
Two pincers
Scissors / knife
Lighter
Open the roof till the first part is vertical. From the middle bar you’ll see a black strap running backwards on each side. Remove the strap by bending the metal piece with two pincers.
Once removed, cut the elastic cord in two. Take one piece and push through the black “seat-belt” loop, where the metal piece from the strap was before. Tie both ends together with a reef knot.
I started with no tension on the cord and the middle bar as far backwards as possible. Do the same for the other side.
Close the roof and check the tension on the cord. Open the roof and check if the middle bar flips backwards by it self. If not tie ends closer to each other. Try again till the middle bar flips back by it self.
Once happy, cut and burn the loose ends. Mission accomplished.
Good luck.