So just fiddling around with the parts for my new build, and it appears that the Aero Precision MLOK handguard I have is contoured in a way that doesn't drop off enough beside the top rail to give the front MBUS sight room to deploy cleanly.

Meaning it gets stuck about halfway deployed and then creeps forward the rest.  

Anyone else have this issue with other handguards?

Basically you can't pull down the levers on the side far enough for a clean deployment because it hits the handguard.

The rail doesn't have as much clearance as on some hand guards. We ran into this on my son's rifle. Look to where the bottom of the sight hangs up on the rail and lightly file it until the sight opens with no resistance. It took a very small and insignificant amount to get it to work.

You can push the top of the sight down to deploy it. Or buy a different sight.