I vividly recall an incident where Jio's CGNAT was dropping idle TCP connections after 10 seconds of idle. (They fixed it, but I don't know if they fixed it for all destinations or only special destinations).

And, of course, I say dropping, rather than closing, because NAT usually doesn't send FINs to close sessions when they're dropped. So you only find out when you try to send more data. Some NATs don't even send RST when you send data on a connection it dropped, so you have to wait for a timeout.

There's also a capacity issue. If you're serving your website via a single IP (common with load balancers), you can only have 65535 connections from each client IP (assuming https on port 443 only, using non default ports is often a non-starter); if the user's ISP is sharing a very limited pool of IPs with a large number of users, it's conceivable that all the connections to your site could fill up.