I tried searching to see if this has been reported but didn't see it, so my apologies if this is a repeat. Also, might just be a Chrome issue, but in any case:

I use the Activity link often to check in on threads I've posted to. As a simple example, let's say thread "A" shows 5 unread (Green 5) and thread "B" shows 2 unread (Green 2). I click on Green 5 and as screen is transitioning to thread "A" I can see the Green 5 go to Grey 0...all is good. Now I hit the back browser button, and "A" still shows Grey 0...all is good. I click on Green 2 to go to thread B and I can see the Green 2 go to Grey 0. Now when I click back from B, the Green 5 is back on thread A even though there are no new posts. If I hit my browser refresh, the Green 5 goes back to Grey 0. I'm a bit OCD and I like to see all Grey 0's so I probably notice this more than others

That's a separate issue and i've seen that too....Robin explained this one in the Martian thread....Because of duplicated posts that are deleted by Moderators the pagination gets off on long threads....So you're probably seeing the problem on the Skit-B or Jpop threads.

The issue I'm trying to describe is the green animation that highlights the number of unread posts...it goes grey then back green again on subsequent use of browser back when no new posts have occurred.

Yes still happening....I had 4 unread posts from amh spooky thread and 1 unread post here...clicked on green 4 and read posts then hit back on browser...4 was now grey 0. Clicked on green 1 to read your post...clicked on browser back and spooky thread back to a green 4. Hit refresh and spooky to grey 0.