Yup - well-spotted. That is definitely a bug.
Those responsible have been soundly chastised. It'll be fixed in the next service release, which is due in a few weeks. I know it'll be fixed, because I tracked it down and fixed it this afternoon, after chastising myself.
You probably dont want to know that it was a 1 bit interpolation inaccuracy in the gamma-corrected layer-blending code. So Ive increased the interpolation values from 8 bits to 16 bits.
I wish they were all that easy to solve!
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"