If I were to guess, there is a basic shell of the app that’s cached or saved on the device, so that’s probably what’s opening and what stores basic items like the logo. The rest of the app is probably operating through underlying api calls between the app and and glide servers. So as I picture it, your custom url loads the shell of the app and then the app directly communicates with glide servers without using your custom url.
When you killed off that custom url, the shell is still cached or saved to the OS/Browser. The shell still has direct communication to glide, so if the app still exists in your account, then the cached/installed app can still communicate with glide. I would guess that glide updates the shell with the underlying glide url that’s needed to communicate with the server…so switching the original glide url to a new app would still allow the cached/installed app to communicate with something. The shell may be installed using old logos, but the rest of the internals download from the new app when you open it.
Still not an answer, but that’s how I view it. I don’t really know how the underlying structure to handle that communication and data transfer works. I would guess that changing the glide url would be enough once you’ve removed the custom domain, but I’m not sure. Definitely a unique situation that most people probably don’t run into.