🇮🇳India @malhar_c

Don't know if this is relevant but here's a quick fix that worked for me. I'm on Drupal 10.3.1 though, and I faced this exact warning while customising the default user login page using hook_theme_suggestions_alter() and hook_theme_registry_alter(), but this might help others as well, if you are in similar endeavour.

<?php
$theme_registry['page__custom_page']['type'] = 'base_theme_engine';

Setting the type key for the theme suggestion directly in the theme registry resolves it.

Cheers :)
Malhar