Dating a divorced Dad

So I haven’t been divorced but am currently dating someone that is divorced. When we first started talking he told me the divorced because she had been unfaithful to him a few times and she actually ended up marrying one of the men she had an affair with...a few months after they divorced. Anyway, he said it took him a long time but they finally got to a point where they’re both really good friends and cordial for the kids. I didn’t think it was weird at first; I admired him for being so forgiving. As time went on it started to bother me a bit more because they text all day, she talks about her marriage issues with him, and then I found out they’re still sharing a bank account. I expressed my concerns and he started to back off from talking so much, but he still is lingering on to the bank account. He explained it as why poke the sleeping bear and how is it affecting our relationship. I also recently found out that he is going to propose to me. She found out somehow (bank account I guess) and blew up. Posted it on Facebook to go congratulate him and tagged him in the post, then messaged me to tell me she was done having small talk with me and congrats on the proposal. I responded back but she had already blocked me.

This week he and her are back to being good friends and texting all the time. I just don’t get it other than I understand being cordial for the kids. But this is all new to me and I am not sure now think/feel. Sometimes it feels like he’s still married to her. I love him very much, my daughter loves him. Any advice?? Or red flags?