As far as I'm aware, this affects all GrapheneOS devices. In theory this is unfixable, and the location tags do show up just fine while logged into the same account on other devices.
My best guess would be that Pokémon GO does not know how to display the location tag because GrapheneOS does not use Google's location servers (correct me if I'm wrong), so it prefers to show a placeholder (usually "Tokyo, Japan") or show nothing at all.
Reporting this issue to Niantic will not do anything because it's a very small subset of players complaining about an issue that does not affect the core gameplay. As a hardcore player this does bother me, but everything else works just fine, including distance searching, so I just learn to ignore it. In a way I kinda like it because it has less text to load when doing Pokémon appraisals.