AFAIK, all the major tcp/ip stacks do round robin port assignment. Given relatively short, and relatively few, connections you should not have any collisions.

If your customers are behind CGNAT, and you have a good number of them, it would be pretty easy to run into collisions. CGNAT doesn't have any problem using the same source ip:port for connections to different destination ip:port, because there's no reason not to.

I agree though, that most OS stacks are very conservative about using the same source ip:port for connections to anything else. It works ok until you need to make lots of connections, then you have to manage ports in userspace.

fwiw CGNAT is fairly uncommon in the United States except for cellphones. This creates one of those bad situations where you can have something that falls flat in the face of CGNAT but also "works fine" for all of your developer/employee traffic.

Imagine the client is actually two machines behind a NAT (corporate network or school) which each open 1 connection.

You can even reproduce this easily with two docker containers on the same host and curl using the "--local-port" option.