Glitch/bug/weird achievement behaviour on Steam?

Yup, it's completely broken. Mine doesn't even show up anymore, so I guess they are already fixing it.

https://steamcommunity.com/stats/BatmanArkhamCityGOTY/achievements/

As you can see my friend, on the bottom it shows 0% for those 2 Batman achievements and yes, I remember that game very well, one of them is story related within the first few missions, so your observations do have ground for concern. I hope its temporary and due to some server memory issue...

Suddenly got a TF2 achievement and Achievement Hunters in showcase.
Scratch that, used to have Achievement Hunters but now replaced with a Batman Arkham Knight one.
Some correct ones still remaining though in my showcase.

It started about 3 or 4 days ago and was reported but dont hold you breath for a fix.
When those showcases were first released there was a bug that caused some people to have an average game completion of 0%.
Despite many reports it was the community to solve what cause it and how. It even got to the point of teaching Devs how to not to trigger it and sometimes even fix it. After many months valve finally got around to correcting their calculations and resolving the problem.

yeah as azatothAU said I noticed it 3 days ago. Didn't know about that other stuff though since I've still barely been on steam compared to others. It does really love having binding of isaac as rare though. At least its not double achievements from pixel puzzle ultimate now

Found some news on this issue:

https://steamcommunity.com/discussions/forum/1/1697221160903242085/?ctp=18#c1697221160916755722

TLDR: Valve is aware of issue, knows what went wrong (corrupted database, old backups), is working on a replacement to repopulate the achievements, will roll it out eventually, and will keep the thread informed of progress.

p.s.
There's a later post saying they started rebuilding the database on Nov 6 and if everything went to plan it would take a week or so. Guessing it didn't go to plan or they underestimated the time.

p.p.s.
It appears to be fixed now, I checked my profile about an hour after posting this comment and my rarest achievement showcase now appears to be correct.
newer post: https://steamcommunity.com/discussions/forum/1/1697221160903242085/?ctp=23#c1661194811511756858

My Achievement Showcase is showing a 2% achievement for a removed game, but those achievements should no longer count or be shown right?

I used to have a full showcase of 0.10% achievements, now I have one 0.10% and the rest in the showcase are 0.30%. Not sure if there used to be an error or if they're still rebuilding their data.