This issue is part of the "flat_foo" sequence, LWG 3786, 3802, 3803, 3804. flat_set, flat_multiset, flat_map, and flat_multimap all have implicitly defaulted copy and move constructors, but they lack allocator-extended versions of those constructors. This means that the following code is broken:

// https://godbolt.org/z/qezv5rTrW
#include <cassert>
#include <flat_set>
#include <memory_resource>
#include <utility>
#include <vector>

template<class T, class Comp = std::less<T>>
using pmr_flat_set = std::flat_set<T, Comp, std::pmr::vector<T>>;

int main() {
  std::pmr::vector<pmr_flat_set<int>> vs = {
    {1,2,3},
    {4,5,6},
  };
  std::pmr::vector<int> v = std::move(vs[0]).extract();
  assert(v.get_allocator().resource() == std::pmr::get_default_resource());
}

pmr_flat_set<int> advertises that it "uses_allocator" std::pmr::polymorphic_allocator<int>, but in fact it lacks the allocator-extended constructor overload set necessary to interoperate with std::pmr::vector.