I forgot to clarify: The issue I am having is NOT it booting into the wrong slot. Eg, it doesn't boot the stock firmware like you would expect it to do if it's loading the wrong slot. What I am experiencing is that it does not boot at all, and simply boot loops. A debrick via tftp is needed to get it to work again. Which is super weird, since it does boot successfully once after flashing. It's just that each subsequent boot no longer works.
I have bought a second device and flashed this one with Openwrt as well via the exact same steps, and this one is working fine. So it looks like it's not that I am doing something wrong.