The GRUB background image says Debian (and has the Debian logo). It should be branded as PureOS.
There might be other incorrectly branded areas in the OS but I wasn't able to go much further, because after a clean install of PureOS 3 alpha 2 in GNOME Boxes, as soon as I applied all updates (using gnome-software) the system wouldn't boot into a working state anymore (no icons anywhere, unable to launch any apps from within gnome-shell, etc.)
To solve this we will need to get rid of any dependency on the desktop-base package of the PureOS default packages - as I recall from Tanglu, this required touching quite a few packages.
Btw, the bootloader should not be shown at all on bootup, and only if the previous boot was interrupted or shift is pressed while booting up or another non-PureOS OS is installed.
by default we now don't see grub (ULTRA FAST TURBO BOOT MODE THAT ROCKS YOUR WORLD) so I think we can happily close this bug :)
GRUB is in fastboot mode, so you should only see it when the install fails. And if you see it, it should have no branding at all :-)