I'll remove this post if 343 decides to fix it.
There have been warnings in some other games' guides sections so I thought this was appropriate to post here and could save a lot of frustration.
I was all the way up to dawn in halo 4 and when i resumed later it booted me all the way back to the start of halo 1/CE.
No I was not ignorant enough to start another cross game playlist, this is a known bug.
Do not attempt this achievement until you're sure that 343 has fixed the issue or lets us resume on any level we haven't finished.