Hi and welcome! Your cargo cover should stay put if it is installed correctly, check to make sure that both of the extendable ends are released. If only 1 end is released it will fit loosely and fall out, also it will be off center so it's easy to see if this is the problem.I just got a 2020 Ascent. The bar slips out whenever I pull the cover out. Maybe a solution is to put rubber pads on both ends.
This is pretty much how any cargo cover I've had contact with works over the years. If it's snapped into place, it's usually pretty hard to displace!^^^
I don't have one in the Ascent but the Forester's are spring loaded on each end.