I also just completed ME 1 Legendary on Insanity and didn't change the difficulty at all for my playthrough, but I only got the achievement for completing on any difficulty, not the one for Insanity.
I had also encountered the Spectre gear bug and used the save file posted on NexusMods to load into a career just before becoming a Spectre as a workaround, and it appears that save file was created with (and loads into) Casual difficulty. Yet, the career save files in which I actually completed the game never changed from Insanity.
I'm sure I'll be playing this again in the future, and I always play on Insanity, but the fact that I didn't get the achievement this time around (potentially due to a workaround for a bug) is pretty frustrating.
Edit: Someone on reddit suggested I replay the final battle with Saren, so I just did that. This unlocked the Insanity achievement for me. Maybe it'll work for others too!
