How does C++20's memory model differ from that of C++11?

C++11 introduces a new memory model that let the abstract machine "running" C++11 code has a notion about multiple threads. It also introduces a set of memory orders by which memory load/store operations abide.

The wikipedia page of C++20 says that it has

a revised memory model.

The reference it gives says that the memory model of C++11 has a number of flaws, which C++20 will revise.

Could someone please give some examples about the problems that come with C++11's memory model, and how that in C++20 would fix it?

Related question: Introduction to C++11's memory model