I manage multiples accounts and identities.

Let us say as accounts:
1. foo@domain_1.com - IMAP
2. goo@domain_2.com - IMAP

Let us say as identities only (set under accounts)
3. foo@other_domain.com - None
4. support@domain_2.com - None

foo@other_domain.com is an alias pointing to foo@domain_1.com, and
support@domain_2.com is an alias pointing to goo@domain_2.com.

All of them have different signatures set.

Behaviour expected (and as it worked until 2.26):
- When I replied an email sent to foo@other_domain.com, the 
  remitent (From) was automatically set with the proper identity,
  which is foo@other_domain.com.

Current behaviour (2.28):
- When I replied an email sent to foo@other_domain.com, the 
  remitent (From) is automatically set with the main account,
  which is foo@domain_1.com.

As user, I would to reply with the same address when the email 
was received.  In special cases I would want to change, but those 
are exceptions, not the rule.