Multi account sso (MAS) login does not work

[RuthgerD]

Describe the bug

I am logged in to my own homeserver via MAS, trying to add my secondary (matrix.org) account fails silently.

Reproduction

1. be logged in with original account
2. • add account, go to login page
3. "Continue with SSO" -> auth.matrix.org says "Continue to app.sable.moe?" (press continue)
4. Transfer back to sable
5. matrix.org account not added, simply shows original account as if nothing happened

Expected behavior

new account added

Platform and versions

Latest chrome, running as a PWA.

Additional context

This used to work with plain passwords