native MPI reduce not triggered

Reported by @Mattijs

in symmetryPlanePolyPatch.C there is a

if (returnReduce(size(), sumOp<label>()))
{

This does not seem to trigger the native Foam::reduce with sumOp<label> in mpi/UPstreamReduce.C ? I was checking all invocations of MPI_Send/MPI_Recv.