I've seen this discussed in the context of a couple of other questions but not on its own. I'm finding that randomly I'll revisit a question where I've left a comment in the past, and be surprised to find that someone replied to me - I was just never notified.
I have not noticed this in relation to comments against answers I've provided, only comments against my comments. This isn't across the board (like I've opted out of the new Inbox or anything), as I do get some notifications - just not all.
Has anyone else noticed this behavior? Is this covered in another question I wasn't able to find? Is this intended behavior?