Anyone ever have an issue with a comment cutoff for notifications?

Use case: today, a user made comment #27 in a long-running thread. This comment did not make it into the notifications for either intended audience, nor to their email notifications. The rest did.

To make the mystery jucier:

1) I am seeing the comment in question on the MyWork "recent comments" section on the left in Classic.

2) I am seeing a thin blue line around the last comment that worked.

So I am wondering if this is a notification limit, if it's a bug, if anyone's seen anything like this before. Thanks!

Attached is a screenshot of the blue box. The comment below is what no one can see in Notifications or email, unlike the other 26 in the thread.