I did a test run on 32 and 24 processors, didn’t see any problems.
The configuration used is:
setenv ARMCI_DEFAULT_SHMMAX 2048
setenv MA_USE_ARMCI_MEM Y
I assume you used two nodes, each with 12 cores. One option would be to increase ARMCI_DEFAULT_SHMMAX to 3072 , giving it a similar SHMMAX to core ratio.
Another test that would be worthwhile pursuing is running 8 cores per node.