I 100%'d all the challenges in every area but the achievement doesn't trigger for me at all. In the Galaxy 2.0 client, it doesn't even show the player percentage who've acheived it, and I just know someone else got it well before I did.
Is the achievement bugged or is something wrong on my end?

I used the demo save file (where I already completed the first Abduction-challenge) if that has anything to do with it. I have every other achievement except this one, so it doesn't sit right in my completionist brain.

Any help is appreciated!

I have the exact same problem, I hope this will bring it to attention so it can be fixed

I've been in contact with GOG's Support and they were extremely kind and manually unlocked the achievement for me. I was most likely experiencing a rare bug of sorts... most likely because of the demo save. (IMO)

If you're experiencing the same issue, please contact Support with proof that you've completed all the challenges (I recorded with OBS, hovering over each area in the map), a screenshot of the achievement shown in your GOG client and, preferably, the save file, should they have any use troubleshooting with it. Your save files are located in:

%USERPROFILE%/AppData/Local/DH/Saved/SaveGames

Of course, you should report the bug directly to THQ Nordic through their Redmine forum, but this didn't work for me. Gave me an "Internal Error", so I moved on to GOG Support. Please try for yourselves if you have the time:

https://redmine.thqnordic.com/projects/destroy-all-humans-community/issues/new

Hopefully, this should work for you guys, as well.
Good news is that I can now move on from DAH! and get to the other games in my backlog. Once again, Support is the MVP.

It's definitely not related to demo saves. I never played the demo, started fresh on day one of release, got all the other achievements and noticed this one was missing from my unlocks. I definitely have three stars in all of the challenges. Game is 100% complete. Achievement is, unfortunately, very bugged.

Do I have to take all of the probes for the achievement?

Just collected all the probes here and nothing, so I guess they're not involved in the achievement and I still don't have it, so I just sent a message to support.

Just an update, the achievement is still broken as of now. I got all probes too and also tried to get 3 stars as the first ranking for the last challenge. The only idea I'd have left is to ace the last challenge on the very first try (because I failed the challenge prior to acing it). Don't have a save file for testing that though. I tried all available versions and I couldn't unlock it:

1.4 from 2021-03-04 (current)
1.3 from 2020-12-17
1.1a from 2020-08-12
1.1 from 2020-08-06
1.0e from 2020-07-28