If an extension provides a PrimaryAuthenticationProvider which supports to link to existing user accounts, it has to supply an AuthenticationRequest that describes the form fields needed to link an account, This can be a button only, e.g. a submit button, in which case AuthManagerSpecialPage will omit the default submit button of HTMLForm. To identify, if a default submit button is needed or not, the form descriptor is searched for containing either HTMLSubmitField or HTMLInfoField (in which case it's assumed that no default submit button is needed). Unfortunatly, the list of classes is hard coded, which means, that other classes (which probably just inherit from one of these classes) can't omit the default submit button, even if they don't need it.
To solve this problem, it could be a solution to add a special key to the formdescriptor array (e.g. dismissAuthManagerSubmit), which, if set, also let AuthManagerSpecialPage omit the default submit.
I ran into this problem in: https://gerrit.wikimedia.org/r/#/c/289099/34/includes/GoogleLogin.hooks.php
How it looks like after applying the GoogleLogin change:
While it should look like: