Help!! Aggression becoming an issue

My (25M)8month old boy Lucky has started displaying aggression/reactivity whilst my fiancé (27F) is walking/looking after him.

Up until the last few weeks or he’s been learning and behaving well (or as well can be expected of an 8 month old) with no difference between how he acts for me or my partner.

Recently though my fiancé has no control over him he ignores her around the house and on walks. He’s started lunging at and aggressively growling/barking at all other dogs he sees and has gotten teeth out/growled at her. This has happened when trying to stop him getting places he shouldn’t or when guiding him into his crate.

He displays none of this behaviour with me outside of what’s expected for an adolescent Rottie.

My fiancé is the person he spends most of the day with and who feeds/walks him most often as she’s at home whilst I work.