We are seeing an odd error with certain users who are trying to log in to a Drupal 9 site. They are using the login form at /user/login, at which point they submit and are redirected to a URL like:
/user/form_action_p_pvdeGsVG5zNF_XLGPTvY....
Which results in a 404.
This only happens to 2 out of 20 users. If I use Drush to generate a login link, it works fine.
What could be causing this? There isn’t much installed besides Pathauto, Token, Redirect, and Acquia Site Studio.