Two Readers Disagree

Four threads, each running once. a and b are independent atomics that start at 0.

std::atomic<int> a{0}, b{0};

void w1() { a.store(1, std::memory_order_release); }
void w2() { b.store(1, std::memory_order_release); }

void r1() { x1 = a.load(std::memory_order_acquire);
            y1 = b.load(std::memory_order_acquire); }

void r2() { x2 = b.load(std::memory_order_acquire);
            y2 = a.load(std::memory_order_acquire); }

A run reports:

register	thread	reads	value
x1	r1	a	1
y1	r1	b	0
x2	r2	b	1
y2	r2	a	0

Reader 1 saw a change and b not yet, so it concludes the store to a happened first. Reader 2 concludes the opposite. Is this outcome permitted?