It's only awkward if you let it be. I have a very good friend who is an ex because they wanted to move to another continent and I didn't. Foolish in hindsight but whatever. By total circumstance, I'm now dating one of their ex's. That pairing split because of different emotional desires. We all hang out when we're in the same country, hell, the four of us (our shared ex married after moving) are going to the sauna together in a month or so.
It's all very normal as long as you let it be.
The odd circumstances aren't really relevant, though I admit they're a bit odd.
If you break up on good terms, I think it's healthy to stay friends. This person has valuable insight to who you were at a certain part of your life and unique perspectives.
You gotta be able to be respectful, though, both you and your ex. Don't weaponize the intimate knowledge you have with each other to score points with their friends.