When you create an opp, as soon as you set the customer, the contacts are auto sourced. When doing an import, doesn't seem to pull in the contacts from the customer. Is this by design or something I might be missing?

I'm not familiar with Opps at all, so I don't know if that's out of the box functionality. If it's not, it could be a script/workflow in your system that's doing that. Did you ensure you have the "Run Scripts and Workflows" box checked on your import? If it's a custom script doing that and you don't check that box on import, I could see what you're describing

The standard behaviour when you create the opp, is that the contacts from the customer are immediately sourced over. I've tried importing with and without the server/workflow triggers. Will check if there are any other scripts deployed in case they are interfering...thanks for the suggestion!