Now I'm definitely sure the issue doesn't lie on the OS. I booted Ubuntu from my USB drive and flashed the PureOS image using BalenaEtcher but the phone booted the postmarketOS beta yet again. Ah!!!
Did you ever get this to work? I use etcher if I am flashing the SD card from Windows but if I am flashing from Linux I use the 'Disks' app.
1. Unmount all partitions of the SD card. (I usually end up deleting all partitions and reformatting the whole card but that is how I roll.)
2. Extract .img.xz to just .img file (Unlike Etcher which uses the zipped file)
2. Select SD card in 'Disks' app.
3. Select Options icon in the top right (I guess that is what you call the three bar icon) and choose "Restore Disk Image"
4. Select the unzipped .img file, select restore, restore, then enter your user password.
I finally was able to install another OS to my phone after flashing JumpDrive to the micro SD card using the PinePhone. Now that I think about it, the problem probably arose from the fact that I was using UMS Enabler on an Android phone so Windows and Ubuntu could detect the micro SD card.