This isn't an issue for the installer per se but rather for testing.
The README instructions currently refer to creating an image and then setting it up as a loop device. But such device can't be listed by parted and therefore also not pyparted and archinstall.

https://git.savannah.gnu.org/gitweb/?p=parted.git;a=blob;f=libparted/arch/linux.c;h=ccbba8656bc7ef0663aea6431bfb8873deccd0ef;hb=HEAD#l2304

Maybe a question for @Torxed would you be aware of other options to get a test device setup that could be used in tests?