Hi Andrew,
I can confirm from source code review that , the support for EnableCustomExprOnly registry is indeed disabled in 12.6
This has not been documented in the bookshelf.
As per the source code comment, this was temporarily disabled for testing the 64 bit support for Policy server.
I reckon they forgot to undo the debug changes.
So this really is a bug.
I have instructed the assigned engineer of your support case to engage SE and provide dev fix.
Cheers,
Ujwol Shrestha
Ujwol's Single Sign-On Blog