We have to set the matrix from which the second matrix will receive numerical transpose data with a call to MatTranspose(A,MAT_REUSE_MATRIX,&B) when B was not obtained with MatTranspose(A,MAT_INITIAL_MATRIX,&B)

We have now to call MatTransposeSetPrecursor prior to MatTranspose in the case described previously

see https://petsc.org/release/manualpages/Mat/MatTransposeSetPrecursor/ for more details