First, can you pump a set of 10 bit colorbars through the process (to see how they get degraded.)
What's your setup for Premiere Pro sequence type? There's no DPX preset - are you using your previews there by any chance? (I'm wondering if things are getting rendered 8 bit there.)
I suspect that you may have to turn on maximum preview quality and have a custom 10 bit preview codec (not sure if both...probably more the former) to pass 10 bits to Encore.
Have you tried testing a minute (or so) with MPEG-2 which is also a valid bluray format?
Have you tried just using Media encoder and bringing that material directly to Encore?