Delayed achievements?

I am running into an "issue" where achievements are not unlocking when they should. I was reading that some of the gears tactics achievements are buggy, and it has to do with internet connection. It may be because I am staying in a hotel and the internet connection keeps going out, but I had just completed C3 A1 and did not get the dead men tell no tale achievement, I had also killed the deviant disciple and did not get Zed's dead, baby.

However, when I logged back on, both of these achievements popped up for me?

The One/Series achievement system is always a gamble but the system (and certain games) have some failsafes built into them. Achievements are not unlocked on your machine but on the achievement servers, your system just sends a message asking for an unlock. If there is any kind of issue with the communication the message may not get through but the system THEORETICALLY keep trying to force these through later. That is why when you log in later it may pop even if you play a completely different game or no game at all. Sometimes it could help if you open your full screen achievement list on the console and open the full screen list of the specific game. That is also forcing an unlock for stuck achievements. Sea of Thieves for example forces stuck achievements every time you login a server (based on the internal tracker) so it is one of the safest games to play on Xbox when there is any outage.

I wanted to test it out and it is 100% the internet (basically what the above post stated). If you are not connected to the internet, the achievements will not unlock at the time of earning them, they will unlock once your internet connection is established (or re-established).

I had ensured I was not connected to the internet, and I played a level. I know I had completed the requirement for Up Up Down Down, yet no achievement unlocked. This morning, I connected to the internet and upon entering the game I saw that I had Up Up Down Down unlocked, and the time stamp was today's date not yesterday's.

So, I believe my issue was the poor internet and when I logged back on that is why both dead men tell no tale and Zed's dead, baby unlocked when I signed back on.

The crazy thing with that is does that mean that technically someone could play this game 100% offline then once they had completed all of the requirements for achievements, establish an internet connection and all the achievements would unlock at once?

Yes it is possible although it is not encouraged to play offline as many games will straight up not unlock achievements later. You are either online when they trigger or you have to replay that section (if possible).

Either way, I am glad I figured out what was going on. The place I was staying just had internet that would cut in and out and once internet connection was re-established the achievement(s) would unlock.