If an attacker controls a subdomain and can trick a user into visiting it (e.g. evil.example.com), the cookies may be out of scope, but the password manager may (or may not) treat the subdomain as part of the domain in terms of .well-known/change-password requests, allowing a subdomain to redirect the password manager and potentially stealing credentials.

It is undefined behaviour. The spec is under-defined. That's my issue, there has been no security pass of this at all. It is left up to each individual password manager to make this secure (or not).