Ah I think Windows does this “helpful” thing where it installs its bootloader into the ESP of any drive if it’s already present rather than the drive you explicitly told it to install onto.
You didn’t have anything in it yet, right? Unplug all other drives and then re-install Windows onto the drive. It should work as expected after that.
IIRC Pop!_OS sets the systemd-boot timeout super short; you have to hold a key after the firmware is done or something to get to it reliably or simply increase the timeout (1s is enough, I have it set to that on my systems). systemd-boot should give you the option to boot any windows installation though, it can auto-detect them.