This IS a miss-able achievement due to a bug (according to the developers). Unless you have a save from before the triggering event, you are going to be in for some pain since you may need to restart completely. As of the patch prior to the October 2023 patch, this bug was present, and according to the changelog for the October 2023 patch, it has not been fixed yet. Rockfish have promised to fix the issue so this may not be relevant forever.Note: I have the PC version of this game, but the basics are exactly the same for consoles, and the current guides are incomplete given the bug.
Cliff notes:
There is a mission called "The Emissary", the triggering Event is at the end of this mission, when you return to Home Base.
To get the achievement, DO NOT
DEFEAT THE EVENT (3 rooms + 1 boss room)!
You may notice a "resolve" bar appear on the right of the screen when you get dumped into the event: let that run out without dying first, I found cruise boosting around the perimeter worked perfectly. Once Resolve hits 0, presto, you get the achievement.
I can reveal that without spoiling anything of consequence other than mission names.
If you have saves from "X marks the Spot" or "Picking Up The Pieces" those are 2 missions just prior to The Emissary and thus the triggering event, but you'll need to put in a bit of leg-work to get back to the end of The Emissary.
Note that many will not be able to defeat the event given their progress in the game, hence why most don't miss this achievement. But you
can defeat the event at this point in the game, it just depends on how overleveled and/or overgeared you are. Once you have the achievement, if you can defeat the event, just load a save prior to the triggering event and proceed with the event as if you intend to kick its butt.
For more details, see below spoilers.
*** Spoiler - click to reveal ***When you Spatial Bypass back to Home Base after the end of The Emissary, an Ancient Rift will open up behind you. It will suck you in, starting a Lunacy 150 Ancient Rift. As you gleaned from the cliff notes, you beat the Rift, you do not get this achievement for your trouble.
You instead get Rift Runner, a much easier achievement that can be gotten at any point by popping open and defeating any Ancient Rift.
To be crystal clear, for the moment this achievement can ONLY be gotten by resolving-out of this specific Ancient Rift, hence the comment on the other guide wondering why he's unable to unlock this achievement, and the background whining on the Steam forums about this issue. The Steam people seem to be fixing the problem by borrowing other people's saves, or digging through their own unintentionally massive libraries of autosaves (speaking of bugs) to find a sufficiently old save to go back and do it again.
According to the developers (found on the Steam forums), there is likely a bug which prevents multiple achievements from unlocking at the same time, and this is apparently the only edge case in the game where this can happen consistently, since defeating this specific Ancient Rift would grant, in theory, both "Rift Runner" and "Still In One Piece" at exactly the same time.
Now, I should note that unless you're overgeared and/or overleveled there is a good chance you won't have this problem: you will not be able to sufficiently dent enemies quickly enough to defeat the Rift, you'll get yanked, and the achievement will pop. Given how easy it is to use high-risk areas to level up and get really good gear however, it's not as hard to be strong enough to miss the achievement as you might think.
When I got sucked into the Rift the first time, I happened to be 30 and sporting a silly Vindicator build that I later determined could waltz through Nightmare without any changes at all, so the Rift got stomped and I continued on none the wiser until I noticed a secret achievement was still locked for some reason after beating the game. You don't need to be anywhere nearly that strong though, a well put together T4 ship with a halfway competent build is in good shape to do a lunacy 150 and live to tell the tale.