I'm pretty confident it wasn't due to a secret relationship. He's a digital nomad so he's never in one place for very long, which he'd stated early on as to why he doesn't expect to have a long term relationship with anyone.

So in fairness to him, he did try to manage my expectations. I knew it was a "situationship" and even told myself to not get too invested, just enjoy the moment with him etc and be ready to move on once he left.

I lost sight of that the longer we spent together, and despite what he said at the beginning, I naively thought that things were naturally developing into something more serious and that he'd be willing to do something long distance with me.

But in fairness to me also, he did say things like how he didn't expect that we'd become so close, that I was the first person to make him reconsider moving back to his home country, and he did all kinds of things that imo, most people wouldn't do for someone who's just a casual fling. It's just hard for me to understand that sort of inconsistency, but I guess all I can do is accept that people can be like that and that they'll disappoint you as a result.

This sounds a lot like someone who is already in a relationship

That sounds a bit harsh. As a guy I can tell you it can simply be due to long distance. If there is little perspective to see each other regularly it's easy to lose interest, especially if it's not your first relationship and you haven't spent that much time together to begin with like in this case