Early morning message/kiss; Nickname that makes him feel mushy with 🥰; Teasing that she must want a hug.
Yes, @Blimey97, your Partner is deliberately flirting. He has begun a romance with this OW. He hasn’t actually been all in with you to work on your relationship because he’s been channeling his emotional energy elsewhere, and she is reciprocating.
Please don’t be conned by his current damage control shtick, where he is downplaying and saying what he knows you want to hear. He is well aware of his intentional behavior to energize their illicit connection and mutual validation. He’s making a mockery of you by expecting you to believe he didn’t realize he was flirting. He is lying.
It seems the flirting only takes place over messages not in person. They’re never usually completely alone in person.
The flirting will also be going on in person via playful banter with each other in the group and more when pairing off for cozy conversations. The truth is you really don’t know what he’s doing when he is away from you. He was engaged in inappropriate chat right under your nose.
As for OW, you’d be foolish to assume he isn’t attracted to her because she is older. Scads of threads here attest to that and I have personally observed it. The chat you saw shows that they are building intimacy and share a frisson. This is not a platonic friendship.
@Blimey97, he’s been playing with fire, abusing your trust, and damaging your relationship. Therefore, he needs to cut contact with OW immediately, as a consequence for crossing boundaries and to make amends to you. That he is refusing that, and is declaring that he will be going out with her, even in a group setting, is outrageous. He absolutely has an agenda to prioritize her and continue building their connection, and she will be expecting that. You don’t have to tolerate his faithless behavior. If he won’t cut her loose, I would end the relationship.