How a GPIO Pin Really Behaves
GPIOA->ODR |= (1 << 5); looks exactly like every other memory write you have ever done, and that resemblance is the problem. On the far side of that register is not a bit of storage but a pair of transistors, wired to a physical pin, with a maximum current, a maximum switching rate, and a real-world net on the other end that may already be being driven by something else. The register model hides all of it, right up until the moment it matters.