I am noticing it quite often, today I decided to watch the counter in the tooltip closely and write down the numbers.

So, immediately after login, the tooltip was showing the following information:

Collected: 38123 / 49206 (77%)
258 things until 78%

I did some random activity for an hour which did not update my collection in any way, and then I handed in the "Hold The Barricade!" quest (https://www.wowhead.com/quest=50177/hold-the-barricade) which has a choice of two items as a reward, with both of them already in my collection in some way or another.

The message in the chat log said I had four new items added to my collection (see screenshot below):

I've checked the addon tooltip immediately, and it was now showing the following information:

Collected: 38013 / 49206
368 things until 78%

So how come that by adding four items to my collection the actual number of collected items decreased by 100?

There could be a few things. Some things don't register from Blizzard until after you log in (Garrison Blueprints) and things. When you log in next do a Shift + Click to force a refresh (after probably about 2 minutes to give Blizzard time to run your background completion checks. Then see if you notice a difference.

If you can screenshot a before picture of your /att and then again after you gain something. This way we can narrow down where the area of problem might be.

The next question is are you using the Live Version (Twitch/Curse) or Git's version? That also makes a difference.

Lastly, we did a fix yesterday that fixed some merging issues in the list that could also help alleviate some of that issue. Most importantly though we need to narrow down the "problem" area in order to see what is causing it.

Thanks for the quick response.

I've already logged off for tonight, but I will do the screenshots before and after refresh tomorrow morning.

As for the version, I am using the one managed by Twitch.

So, this morning I logged in, waited around 5 minutes or so, then did a force refresh and added an item to my collection. Here's some screenshots.

Before refresh:

Right after refresh:

After adding an item (you can see that both a pet and a wand have been added, but I already had this pet in my collection):

Notice that this time the problem with decreasing counter did not happen, but it did not happen yesterday either when I was adding some green item I bought from an auction house. It did however happen yesterday when I received a quest reward, as I described earlier, so maybe I'll just have to wait till it happens again and post more screenshots then.

Also notice that the number of collected things before and after refresh more or less corresponds to the number I had yesterday before the glitch happened, so it seems like the original value was correct, and the one after adding a quest reward to the collection is wrong.

So, I just logged in tonight, checked the /att window - it was in the same state I left it this morning, shown on the last screenshot in my previous comment.

I've waited a couple of minutes, ported from Garrison to the Legion version of Dalaran, and did a force refresh.

The screenshot immediately after refresh:

The counter in the Outdoor Zones and the Achievements category was decreased even though I did not alter my collection in any way between two force refresh attempts.

I will try to post some screenshots later with both categories expanded in case it helps you.

Did some more testing.

Force refresh after login, I am at the Draenor Garrison:

Force refresh in Legion Dalaran immediately after making a previous screenshot:

It seems that not being in Draenor marks a lot of Draenor things as not collected, mostly Garrison-related staff.

This was caused by the Heirloom API returning no information sometimes. We have solved this by utilizing a cache. It will be available in the next Release.