I thought I had a eureka moment for 2023, where I would take a one-way to europe and then book a series of round trips back to the US in order to save a substantial amount of money - but it appears impossible to do this. I thought maybe it was a Schengen/EU thing but I've tried multiple combinations and it keeps spitting out errors.

For example this fall I'm returning MUC-CDG-CVG in late September and then heading back CVG-JFK-ARN a few weeks later. These flights have seats available. I'm already on them, but I can't book MUC-CVG-ARN. Is this a citizenship issue or do they know what I'm trying to do and blocking it?