I don’t get it because he’s actually a lovely kind person in everything else
I think what's happened here, and the reason why you find it so confusing, is because he hasn't changed that much as a person. However, for some reason, you've been devalued and deprioritised.
That's what you're experiencing - this is how he treats women he doesn't really value I'm afraid, and it must be a bit of a shock to outside of that bubble.
If he explained this situation to other men, honestly, I think they would see right through this regular & late night texting.
I agree with @MsDogLady .
Have a look at what I wrote in response to a similar thread yesterday -
https://www.mumsnet.com/talk/relationships/5404974-partner-new-woman-best-friend?page=4&reply=146939533
*Some simply enjoy having a stable of women rotating around them, lapping up the attention, planting little seeds in case he wants to jump into a relationship with one of them one day. It's hard because some liberal and creative types are like this, and they try to claim their female friendships are all part of their progressive, left wing liberal ways....whereas actually they're just like any other average Joe who enjoys female attention.
I saw or read something somewhere on the emotional toll this takes on people. Your body lives in anxiety most of the time because it's on high alert, because emotionally, you don't feel safe.
I think it's a responsibility of partner to actively be mindful of how they can make their partners feel emotionally safe, and not just expect them to trust them, no matter what they do or who they choose to spend their time with.
It's not nice risking being seen by other people or your friend group as 'the jealous one', but for your own wellbeing, I think you need to openly say that you're not a 'cool girl'.
He has the right to be friends with who he wants.....but you ALSO have the right to not be in a relationship that makes you unhappy. If other women are willing to tolerate bullshit, that's on them, and I'm afraid they will learn hard lessons one day.*