I'm aware of corporate MitM proxies, but it's less common to run into these kinds of issues if you simply use wss://, because a) they're not quite as common as

I'm aware of corporate MitM proxies, but it's less common to run into these kinds of issues if you simply use wss://, because a) they're not quite as common as your typical ancient HTTP caching proxy and b) there's a better chance they either support new protocols natively or at least support HTTP CONNECT (like squid).