I bought a Bushmaster HB M4 type upper a couple years ago and have been using a TA01NSN on it until recently. I just put a new Aimpoint and KAC 300m BUIS on it. When I was sighting it in, I had to crank the windage on the Aimpoint and BUIS all the way over to the right to get it sighted in. The flip up arm on the BUIS is almost touching the side of the sight base.
I don’t remember having to do so, but apparently I had to do this on the TA01NSN also, because when I sighted that in on another upper, I had to adjust the windage back to the left.
This can’t be a canted front sight post, because I had to adjust the optics also. What could be wrong? Could the flat top rail have been machined that far off the centerline of the upper? Could the barrel be out of alignment? I’ve never built an upper, but is there a way I could fix this if I get the right tools?

weaver vs picatinny specs on the mounting bracket?  something to look at

Since you really had to crank the optic's windage, too, it sounds like the rail is offset or angled left/right.  (Either that or the other rifle has a screwy rail, plus your FSB is canted on the Bushy.)