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.
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.
Then, push both ends firmly into the recesses, the springs aren't strong enough to get it fully engaged by themselves.
Mine is used daily and has been in place for at least the last 10,000 miles without any problems.
Hope this fixes your problem.