Thresher Maw Achievement Not Unlocking.

Hello all.

During Grunt's loyalty mission, I defeated the thresher maw with about 20 seconds to spare.

The 'Big Game Hunter' achievement unlocked in the Mass Effect 2 achievement list in-game/tracking menu--but the achievement did NOT unlock with the Xbox itself.  It still shows it as being locked and I'm still out 10 points.

I have a feeling I might just be out of luck, but any ideas/suggestions are welcome.

Mass Effect Legendary Edition on Xbox One X.