This appears to be Tether related. I'm still not certain whether or not it is specific to Hangouts; I suspect it is not but the notification has to be clickable.

It is specifically opening Settings > Mobile data.
It stops happening when I disable Tether.

Specific repro:
1. Log in with an account with Tether set up and make sure tether is enabled.
2. Open the Hangouts app and put it in the background.
3. Send a Hangouts message to the account from another device so that a Hangouts notification shows up.
4. Click on the Hangouts notification.

Observe:
1. The Hangouts app is focused but a Settings > Mobile data window also appears.
2. Disabling Tether in chrome://flags prevents this from occurring.

-> khorimoto@ for triage / debug

So far I have only tested this on 61.

What version of 61 is this? This seems like  issue 738225  which has been fixed for a while.

61.0.3136.5, which I just noticed is quite old and my device received an update today and the bug is indeed fixed, hooray!