Subsequent notification emails for subscribed post?

We have a member (an employee) who has subscribed to a specific post, and other than an initial email notification about a reply that was posted, they have not received any additional notifications by email. 

I’ve been looking for documentation, but can’t find what I’m looking for. So, I’m asking here:

• Is this expected behavior? If so, why?
• If not, is the source likely to be a Gainsight bug, an issue with the user (e.g., accidental unsubscription), or something else (overactive company spam filter in response to multiple notifications, etc.)? 

Thanks for any additional perspective!