Does not look like Pure Maps is available from Flathub:
$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
$ flatpak remote-ls flathub | grep maps
Meteo - Know the forecast of the next hours and days with data and maps com.gitlab.bitseater.meteo 0.9.8 stable
Laigter - Tool to generate normal, specular, occlussion and parallax maps for 2D textures io.itch.azagaya.Laigter 1.10.1 stable
Grock - Displays geological maps of the UK. me.leucoso.Grock 1.0.0 stable
Click on the app and select Install.
Wait for installation to finalize before launching.
Available flatpaks are mixed in with regular native apps if you have installed the Flathub repo. You can tell them apart by clicking on an app and looking at its description.
Note that after adding flathub via the command line, I had to reboot the phone in order to get the store app to load the flathub repository. After that, apps like Pure Maps showed up for me.