Having issues with achievement totals

This sounds like my issue.

Some achievements on the XBox One are just not adding to my total gamerscore. It's rather strange, since some achievements do add to my total gamerscore, and it shows that I have all the correct achievements in the individual games.

My total GS is 75 points less than what it should be at the moment.

This is a widespread issue that has been going on for days for quite a big selection of players. There is not much you can do. Some people were lucky enough to fix it by unlocking more achievements in the same game, but that's not possible if you've already
unlocked all the achievements in that game or by simply unlocking achievements in other games... but that's obviously not a real solution. There is a problem with counting the achievements when you unlock them, therefore they don't increase the number of
total achievements you have in a particular game, the total gamerscore you have in that particular game and thus, your total gamerscore across all games. All in all, it is a server-side issue.

My current issue is that I have 75 completed achievements in the MCC for a total of 580 gamerscore. When viewing my achievement app fullscreen (and viewing achievements on xbox.com) it tells me I have 74 achievements for 570, but when viewing MCC achievements
in snapped mode it has the correct totals. I cross checked the achievements from the snapped mode with xbox.com and all the achievements are present in both, just one isn't being added to the tally on xbox.com. Not to mention the 4 or so "unlocking" achievements
from Tuesday still.

There are a total of 10 achievements that should have unlocked. There isn't even progress in the achievement app concerning those achievements.

As others have said, OP the only issues tend to fall with achievements which should need to come online being earned offline.

Order of achievements isn't an issue if they were earned offline because no timestamp is applied. If you take a look at my Achievements for Halo Reach on Waypoint or True Achievements my order is totally off for the single player achievements because I earned
them offline with no timestamps.

Shouldn't be an issue (unless you've done something wrong like modding)