From the information you've provided, I couldn't say for sure that this isn't a bug in ACML-GPU, but we certainly want to take your problem seriously.
I'd suggest you go to the helpdesk http://developer.amd.com/support/Pages/default.aspx and submit a helpdesk request (be sure to select GPU Tools support and ACML-GPU.)
Since the NO_GPU environment variable forces the library to use the CPU instead of the GPU, please include information about your CPU.
Since the problem depends on the array size, please also be as specific as you can about the parameters passed to DGEMM. If you can create a test program that reproduces the problem, of course that would be ideal.