How come

I wont go into my ex. as it's irrelevant, but needless to say, because you've shared, it's a similar story to yours. Also look as an example at Tnr9's story, and there was a post as well by Stu from a while ago, it's an outcome that can happen, the former partner is "seemingly" happy and "different" to when you were with them.
It almost fits into an "up and down" type of dynamic imao. The person is up and down emotionally like a yo-yo, depressed one minute, traversing the globe the next, within months!!!!.
A question to you, would be, have you yourself done this? If you can be mindful of the relationships you've had, would you have come across to an ex. you've been with in a similar fashion with a new partner you were temporarily "enthralled" with?

You knowing yourself as an F-A, how hard all this subconscious stuff is, the mindfulness around emotions and feelings etc etc. Your ex. who I believe has remotely been told about attachment theory and ignored it? What do you think the likelihood of long term success is for any relationship he is in? Another question for yourself. Would you prefer to be in a "codependent" relationship with all the dynamics associated? He showed you his true self ultimately, that's him, so are you willing to put up with that behaviour? I doubt it. You walked away quite rightfully.

As Me says above, different person, slightly different dynamic, and maybe, more along the lines of the "familiar" to childhood trauma. Trauma bonding in effect, fantasy bonding temporarily (maybe even for 1-2 years until the pent up emotions/feelings explode as per usual with an instant relationship break down, as per usual). All the stuff going on in the background that is pent up, not being communicated effectively. Just some other food for thought. If you are truly happy in yourself, busy and have a fair bit going on, you literally don't have time to post up "how happy you are", it's simply not a consideration, you don't think about it, nor need to validate it.
What will be certain is the following, he is displaying his full "fake for public consumption" persona, to his new partner, and the public (including yourself) the one that is placed up to protect his true self from being hurt, this is a known fact psychologically with an F-A attachment.
And I'll tell you from my knowledge of my ex. she has done ALL this before, and every single one of the guys she put the "happy" face up on social media with, admitted to me either "controlled", "manipulated" and/or cheated on her. Go figure! She's doing the literal same thing yet again right now, sadly for her.

Work not done = same outcomes! very simple. Wouldn't concern yourself with what the "perceived" social media photos/statements show. Patterns never change, unless mindful and serious work is done. Alexandra is a prime example of this. You are far better off working on yourself and being finally at peace and happy in yourself, not concerned with "perceptions" of happiness, these other people display to cover for their true lack of self love, partnering to "feel" happy, trying as you know all to well to make up for the void. That's the reality of the situation.

I'm not bothered that he has a gf. He new her for 10 days then went travelling in his van around Europe with her. He always felt so awkward in confined space so its puzzled me how he travelling with a person he calls his gf in such a small van. By the way he is from the uk and she is Canadian and will be returning home to her own country soon. All that makes sense as it will end up a long distance relationship and he has always wanted to live in America or Canada so could be using her. He has never been on holiday with any of his exs so I'm just confused to how he has overcome what appeares to be a classic FA attachment to travel with essentially a stranger as I said he had only known her 10days before they left the uk to travel Europe.

Thank you for your reply and I feel you have hit the nail on the head. Interesting fact. She is Canadian and met him during lockdown whilst travelling around the world in the uk. I had kicked him out for using me and he was living in his van that's how they met. 10 days later lockdown starts to lift in Europe and they go off travelling together. She has to return home to Canada soon so what then. I know all of this only by looking on his social media, it's like hes telling a story he goes into far to much detail it's like he doing it for an audience.

I have moved on from this relationship and would never ever go back there, he knows I'm well and truly done and have moved on. I am dating a secure man and things are going really well for us, he calms my nervous system right down and I have been able to heal and grow more secure with him. I have worked very hard on my own attachment issues and have not been triggered either way so far and I'm able to show up so much more secure than in any of my previous relationships 🙏☺️

Thank you hun, your right just block him on everything and get on with my life with my new man. My relationship with him almost destroyed me as I wasn't in a strong place to begin with. I feel so grateful to have met a lovely caring honest man who wants the same things in the future as me. I hope your ok hun and thanks for all your support over the last few hellish years xx