The standard WP registration form at /wp-login.php?action=register has arbitrarily set tabindex values hardcoded into it; with custom fields, you get an atrocious user experience, confusing the user.
This issue was flagged four years ago already, and declared "fixed" since:
And the same happened again, a mere two years ago:
You'd think someone woke up and smelled the coffee in the interim, instead of succumbing to the reflexive "let's implement some accessibility" fad in a ham-fisted way.
Fix in the interim to hack it out of the core, anyone?