On the exposure tab I am finding that although the RAW RGB value may read 255/255/255, the Destination seems to be limited to 254/254/254.
I raised a support case and the answer received suggested it was down to the profile I'm using. My output destination profile is Adobe 1998. I changed the output destination profile to various other profiles and noticed the destination RGB values jump up to 255/255/255 on some profiles.
Can someone give me a hint here as to why I'm unable to get an output of 255/255/255 when using Adobe 1998.