User's notifications are unexpectedly being marked as "read"

(Version 12.1.4.25057)

I received an interesting message from one of my members today. He is extremely active, consistently one of our top contributors month over month. He's subscribed to almost all of our forums and is a member of basically every available group -- he's potentially more plugged into our community than I am 

He relies on the "lightning" notifications to keep up with the various conversations happening in the community but he has recently noticed he's having intermittent issues where notifications are being marked as "read" for posts that he hasn't actually looked at. His description of the issue:

I typically keep a browser tab open to SugarClub to check throughout the day, but normally only check the tab when I see the tab title indicating there is a new post. I am receiving the notifications of a new post, but for some reason, the notifications are getting marked read without me ever seeing them. Looking at notifications I should have received today, I never saw an unread notification for these posts:

{Example 1: This is a forum post in one of our public-facing groups}

{Example 2: This is a blog post in one of our private groups}

But I did get an unread notification for this post from today:

{Example 3: This is a calendar event in one of our public-facing groups}

All the release announcements yesterday showed as expected. 

I started noticing a trend because I would get a notification for a reply by someone to a post that was a week or 2 old, and the reply was the first response. I had not remembered receiving any unread notification for the initial post. I started watching the home page more over the past week and have found multiple new posts highlighted there that I did not get an unread notification for in the browser.

He and I discussed this, and he went on to also say:

Yeah, I still get all notifications in that lightning bolt, but some are marked as read despite me never seeing them. I get email notifications but I funnel that noise far away from my inbox ;-)

So the notification process itself is working, whatever is tracking whether I saw the notification on the site is not.

FWIW, I'm using Chrome and haven't added any new extensions recently. The extensions I do run are pretty minimal.

I spot-checked my own notifications menu ( {siteurl}/user/notifications ) and didn't notice anything out of the ordinary. I checked his user settings ( {siteurl}/user/settings } and his Notifications and Subscriptions setup all look fine/normal. He's set up to receive Emails and Live Alerts for every Notification option, and he has every checkbox enabled.

Unfortunately, I'm not sure how to validate this issue or where to start any kind of troubleshooting. I checked our reporting, but since all of his examples were from today, I'll need to wait until tomorrow to check if/when he "viewed" the example posts to see if the platform is recording a "view" somehow before he sees it. I also checked the Release Notes for Community 12 but don't see any fixed issues in more recent releases.

Has anyone experienced this before, or have any ideas of what could be causing it?