In the logs you'll find things like:
BLAS_INC_DIR set to /cvmfs/soft.computecanada.ca/easybuild/software/2020/avx2/Compiler/gcc9/flexiblas/3.0.4/include
which should be
BLAS_INC_DIR set to /cvmfs/soft.computecanada.ca/easybuild/software/2020/avx2/Compiler/gcc9/flexiblas/3.0.4/include/flexiblas
to find cblas.h etc, unless pkg-config is used. Similarly the easyconfig/easyblock doesn't set CPATH correctly.
I think foss-2021a easyconfigs that use cblas.h compile anyway since they find it in OpenBLAS' include directory via its CPATH but that's not what's supposed to happen.