The 2004 Alliant data book shows some American Select 1200 fps 7/8 oz loads in Rem STS and Win AA hulls, but only with the Claybuster 4100-12B and Winchester WAA12SL (grey) wads.
The 12S0 is only shown with the faster 1300 fps loads. The pressure is very low with even the 1300 fps loads, so I assume it is a fitment issue, that the 12S0 does not take up enough room.
You will probably have to adjust your crimping stations to fill the hole as you slow them down.
Michael