Domain mapping: Text displayed on wp-signup.php asserts site doesn't exist when it does actually exist

[henry.wright]

In a network install, when domain mapping is active for a custom domain, the following message appears on the wp-signup.php page when the network sub-domain is visited:

The site you were looking for, ​https://foo.example.com/, does not exist, but you can create it now!

Steps to reproduce

1. Map a network site "foo.example.com" to a custom domain "foo.com" using this guide: ​https://wordpress.org/support/article/wordpress-multisite-domain-mapping/
2. Visit the network sub-domain "foo.example.com" in a browser
3. Observe the text on wp-signup.php which notes "foo.example.com" doesn't exist

The network sub-domain "foo.example.com" does exist so this text shouldn't be displayed.

[henry.wright]

Related #53355

[henry.wright]

The behaviour described in this ticket is expected because foo.example.com ceases to exist when domain mapping is made active. Please feel free to close.

See #54325