I had a problem milling a 6"x24x 1/2" plate. nc prog mills .05 off top and periphery, drill and taps a dozen different holes.There are a bunch of M0's to relieve part in vise jaws (3vises). plate starts out at 3/4" thickness and is dusted on both sides initially. After milling ,drill and tap (6-32,8-32,10-32),the part is flipped over and rough milled to thickness leaving .02 for 2nd flip and final thickness.Im using a 3" 4 tooth mill {shear hog ,polished inserts, flood coolant). plate bows about .01 after roughing.I've run these many times in the past without too much trouble.
The stock I got last week warped big time (.03) . The more I mill these things
the worse they get. Would this stock have the wrong temper or be the wrong grade of aluminum ,look the same.

You're putting stress into the part as you machine it most likely. 6061 is pretty stable. I just did a job that was 22 x 10 x 2.5. Cut lots of pockets and never moved. Pockets both sides. Floors finished at .093. Walls were pretty thin as well. about 80% of the material went in the chip bin. You can control the stress to an extent with speeds and feeds. Use radiused cutters too...

This part is generally not a problem.raw stock is suspect though