There are a few different criteria that are checked when determining if an error warrants emitting the sessionExpired event. As a general rule, it gets emitted if a page redirects to /login.
Valve being Valve, they're incredibly inconsistent as to how they communicate that you aren't logged in. The request for getting notifications returns 401, which isn't one of the criteria that emits sessionExpired. It probably should be, but I'll have to check to see if there are any other edge cases that might arise by adding 401 responses as an indicator to emit sessionExpired.