Gears 5

Question: Does the Achievement for completing all chapters pop at the same time?
Answer: Yes, it does, as I found a video of a first playthrough that proved it.
HOWEVER, I discovered a workaround. If you join a co-op session playing through Act 1, Chapter 6, you'll be able to get this achievement without popping the achievement for completing all chapters, so long as you still have at least one other chapter left uncompleted. Doing this also unlocks Act 1, Chapter 6 for independent replay. Do note that there is another progress-based achievement that unlocks early in Chapter, but since joining the session apparently unlocks the chapter for replay, you should be able to back out and go back to Chapter 6 later.

UPDATE: I just went back and loaded up Act 1, Chapter 6 in order to unlock my next Achievement "The Spirit Walk," which popped after the mini-cutscene where Lahni lit the lantern. Both that and the Achievement "Enter Hivebusters" for completing all chapters popped, so my workaround apparently doesn't work, even though I HAD NOT completed Chapter 5. Am complaining to TC right now about this.