So actually even though the forwarding is configured in Roundcube, the mail rules are actually managed and executed by Dovecot (the IMAP service). Roundcube accesses the rules over the “managesieve” protocol. Other applications can too.
The forwarding that is configured in Roundcube is a proper mail forward equivalent to composing a new message, so it should not fail SPF/DMARC filtering, unlike aliases, which do in this scenario.