See https://github.com/leoleozhu/android-captive-portal and https://success.tanaza.com/s/article/How-Automatic-Detection...

If you could return valid internal DNS entries for these domains, and serve up a 204 to the right requests, you should be able to stay online.

You need to ensure you don't do it for every possible domain - I believe Chrome also tests random non-existent domains too, to ensure it gets back the expected error.