A newly installed system! Nothing changed in fstab, only activated the swap partition with sudo swapon, reboot. This can have nothing to do with the fstab …
I guess if you did add the swap partition post-install, a reboot would have just activated it automatically as per systemd activation of swap in GPT disks. With
sudo swapon you activated it for the current seesion.
I have now entered the swap partition in the fstab anyway to avoid future problems - but the solution of @pebcak seems understandable to me.