I had my CS-6 ACR set for 16 bit by selecting 16 bit with an image file open, and selecting done, and for some time it remained that way.
Yesterday, I processed a dozen images through ACR as TIF files---went to work on some today, and found them all to be in 8 Bit Mode.
The only thing I can think of that would have changed the behavior is an ACR Update put everything back to "Default" settings, which is really annoying, as it's cost me a days work!!!
Is there a way to select 16 Bit in ACR, in a manner that remains unchanged unless I choose to change it? And , if not-----why not???
ACR Workplace settings always stay how you set them unless you change them yourself. An exception might be greyscale, but otherwise, I'm guessing you set it to TIFF somehow.