Yea that sounds incredibly frustrating. Glad it held on just enough to not take flight.
I see it's the passenger side mount that has the brace that runs to the other side. Seems like there's still movement causing it to break free. You think adding a second brace, completing the X, would do it?
The spacer may also be adding stress and leverage for it to be pulled out. When you do pick another wing, see if you can find one that minimizes or removes that spacer (you could put the spacer on the bottom mount, or both top/bottom and split the size between them - you'd need new cross braces).