How to approach my (24f) boyfriend (24m) getting too close with a female coworker

Here is the situation, advice on how I could take this forward would be appreciated.

I (24F) feel uncomfortable with one of my bfs(24M) relationship with a female coworker our age. Quick background, we’ve been together for 6 years, living together for 3, in a process of buying a house together, talking about engagement. Communication has always been good.

To sum up, the last few months It always felt like he was trying to find ways of seeing his coworker (24f) at work, and he really enjoys her company. It’s fine to have friends at work but I started getting uncomfortable when he would never mention her name, if he stayed up late after work to talk to her, he would never mention that she was there, in fact he would never say anything about her when I asked despite me finding out that he’d spend the whole day with her. He would look to go in on days that she’s in, always book a desk next to her if possible despite them being in different teams, they don’t only see eachother at lunch (which they always have together), but also any break they have, if they are bored they’d go get a booth to chat and etc. The other week they had a work event in London where both of them and some of the coworkers got to London the day before the main presentation. Me and my bf were already in London with my parents beforehand but he wanted to go and have dinner with them all when they got there (which is fine) and stay in a hotel with eveyone overnight as it’s closer to the office (again, fine). The only thing is, her train (and this other coworker) was getting in at 4pm, where the dinner wasn’t until 9pm as the rest of them didn’t get in until late. He really wanted to go and meet her and her friend at the station as she got into london…I questioned why that was necessary if the dinner with Eveyone isn’t until 9, what he would do for that long before hand and he agreed to leave later… then while they were waiting for the others to get in, he got the drinks, she forgot to send him money for it and when she apologied and said she’ll do it now, he insisted that it was ok and she didn’t have to, (which is a bit iffy in my opinion/ where is a line between being nice but also staying professional), once she transferred, he then replied thank you with a love heart. He never used that before, he doesn’t use it with anyone else but me, I’d don’t think it was appropriate .

I felt uncomfortable with how close they are getting as that’s when feelings begin to develop. I’ve talked with him about it and he said that he is sorry, that’s it’s not what it seems, she’s just a nice friend at work, but he said that he will take a step back and back off and will not initiate conversations or meetings more than they have to for work if that’s a boundary I want to place. I said that I don’t want to be that person who tells him who he can and cannot talk to but how it’s upsetting me as I can see how much he enjoys spending time with her and that’s how feelings can start to develop (if not already – which he denied). He said he will take a step back. He said he didn’t realise he was leaving her name out whenever talking about events and why he stayed late and etc, and that it wasn’t intentional and how their relationship is innocent and he doesn’t seek ways to spend time with her.

Today I find out that when he came into to the office, the first thing he did was message her to ask if she wants to have lunch together and go on a stroll to the shop together. Going completely againsts to what he has promised me, he then also saw her multiple times during the day again to get a booth just to chat, initiated by him and some by her (gathered from teams messages).

I also found out that a social next week, to which he insisted he had to go to (I asked him if he could drive me to dentists as I’m getting 3 of my wisdom teeth removed – to which he said he really wants to go to this social as it’s a small team retiring leaving due thing, ok..), I found out that she will be present there despite him not mentioning anything about her before when I asked who will be there. Again, he just left her name out… This whole situation is making me shake. I have confronted him About it all to which he apologied and said eveyrhing is completely innocent. He thought he told me that she was gonna be at the social before and that he forgot about backing off because it was all innocent.

It feels like he is always continently “forgetting” to mention her in any situation. This was always the case before I had a chat with him the first time… He went against the boundary we have talked about after I told him how upset I was. I know he says it’s innocent but I can see how he is enjoying spending time with her and it doesn’t sit right with me.

I just feel like he prioritised his relationship with her over boundaries that have clearly made me upset and he agreed to set…

I have taken some time away from him and went to my parents for a bit. I must say outside of work they have never met, they don’t text outside of work either (from what I know) and she is also in a relationship. However, any opportunity at work to see her, he is there first point of call (or outside of work during work related events).

Is he getting close to her? How do I take this forward having already spoken with him about it and him not thinking there is anything wrong…

TL;DR How can I approach a situation of my boyfriend getting close with a female coworker despite setting boundaries that were ignored