Forming a route set in an ACK

[rus1g]

Good afternoon. A question about forming a route set in an ack. The code implements an approach in which the route set is recalculated for each 200 response. Although rfc 3261 clause 17.1.1.3 says that it is necessary to form a route set based on the sent INVITE.
https://datatracker.ietf.org/doc/html/rfc3261#section-17.1.1.3 If the INVITE request whose response is being acknowledged had Route header fields, those header fields MUST appear in the ACK. This is to ensure that the ACK can be routed properly through any downstream stateless proxies.
That is, UAc sends a re-INVITE with the route set received in the initial transaction, receives 200 without RR and sends an ACK without route set to the rAddr received in Contact.
https://github.com/sippy/b2bua/blob/master/sippy/SipTransactionManager.py#L497

It's not a bug?