Testcase: science/conduit OPENMPI_CMAKE_ON=mpi:openmpi is supposed to set -DMPI_C_COMPILER:PATH=${MPICC} etc, but it doesn't. The OPENMPI_CMAKE_ON+= line has to add thee values in this port.
I just tried it, with a fresh ports tree (dated Mon Aug 3 09:30:00 CEST 2026), and it works for me. Have you anything else set in /etc/make.conf? Or some environment variable?