Exact URL of where the bug happened

https://lichess.org/training (not logged in)

Steps to reproduce the bug

1. Select difficulty level other than normal on puzzle page (not logged in)
2. Once redirected to /signup page, click sign in button in top right
3. Sign in to account (/login?referrer=/signup)
4. You are redirected to /training/difficulty/mix which is a 404

What did you expect to happen?

Redirected to puzzles page with correct difficulty after logging in

What happened instead?

Redirected to a non-existent page

Operating system

Windows 11

Browser and version (or alternate access method)

Firefox 111.0.1

Additional information

No response