Challenges in female friendships

I've been dating a guy who’s 23 for a year now, and I can honestly say it’s been the best relationship I’ve ever had. He's genuinely the sweetest person, and he has really restored my faith in love and relationships. When we first got together, I made it clear that I was okay with him having female friends, as long as it didn’t cross any boundaries in our relationship or if those friends didn’t know about me. He did have a casual female friend from work who was aware of me, and everything was fine—so much so that we even planned a dog playdate together. However, about two weeks ago, I noticed that he had texted her calling her “hun,” and she replied with hearts. While "hun" isn't the worst thing, her response felt a bit flirty, especially since he was congratulating her on a new job when he said it. I expressed to him that it upset me; the pet name and the hearts felt inappropriate. He insisted that the friendship wasn’t significant and ultimately decided to unfriend her, despite me saying he didn’t have to do that—just that I wanted him to be mindful. So, it came as a shock when I found her in my Instagram suggestions just two days ago. I checked his Instagram and saw that he had added her back. He had muted her notifications so I wouldn’t be aware of their conversations. The messages started with him telling her I forced him to unfriend her (which isn’t true) and then claiming he missed her and wanted to hang out. I was outraged. I woke up immediately and confronted him. Not only had he hidden the notifications but he seemed to be planning to meet her while telling her he missed her. If their friendship wasn't that important, why did he say he missed her? He claimed he missed her because they could talk about dogs and still wanted to make arrangements for the dog playdate, insisting he just wanted to figure things out so we could all be friends. He admitted he was hiding this from me because he thought I’d get upset if he told me he wanted to talk to her. I’m struggling to believe this reasoning. I never told him he couldn’t be friends with her, and we’ve always had open communication in our relationship. So, why the secrecy? He promises he would let me know if they actually hung out, but shouldn’t he have told me he wanted to see her before actually doing it? This feels wrong and deceptive. He ultimately blocked her on all platforms at my request, saying he lost trust in being her friend, but I’m left unsure about everything. I never thought he would be unfaithful or act inappropriately, but this situation has really shaken me, and now I'm questioning what I can trust and whether he's being completely honest.