Definitely not normal on the A850/A900. Check that APS-C size capt. (Item 4 in the spanner menu) is not ON. I doubt that that would produce posterization but I can't think what else would at the moment, other than possibly using ISO 100 as you do. You should know that the minimum recommended ISO on the A850 is 200, not the usual 100 and although there are many different opinions as to what is the best minimum to use (ranging from 160 to 320) I always use 200 ISO.
As I am sure you know, posterization or banding would normally be produced by reducing your number of colours in post processing. That is where I would normally look first for the problem, not in the camera. However you say you are shooting RAW and the banding shows on the LCD display. The display is from a .jpg but is of very high quality, although the engine may be suspect by some. So I am stumped on this one as I have never experienced banding from the A850.
Sony A7, A850, A77, A700 || Sony>> 70-300G, 28-75 f2.8 SAM, 16-80CZ, 50 f1.4, FE 28-70 OSS, FE 24-70CZ f4 OSS, LA-EA4 || Minolta>> 300 f4 G HS, 200 f2.8 G HS, 100 f2.8 (D) Macro, 50 f1.7, 28 f2.8, 28-135 f4-4.5, 70-210 f4, 500 f8 Reflex, TC x 1.4 HS, TC x 2 HS. || Sigma 21-35 f3.5-4.2. || Tamron SP 24-135.