This is a slightly tricky one.
First, if you replace back the Al atom with Hf, then it is indeed recognized as space group 14. And, the structure as such in QuantumATK will correctly be represented as monoclinic no matter what.
However, by changing one atom (Al@Hf as a substitutional defect) you violate the symmetries of the atomic positions to such a degree that P1 is the only remaining space group symmetry. This is because there are so few symmetries in the first place in the monoclinic spacegroups.
So, you actually need to separate the "lattice symmetry" from the space group. In your system, the lattice is still monoclinic, but the atomic positions do not match any monoclinic space group. The same can actually be seen for a cubic case, although the many rotational and reflection operations mean that a single defect only lowers the symmetry but the space group can still be cubic. For instance, you can make a supercell of 8 Si atoms in fcc and replace one atom with Ge, which turns it into 215 instead of 227. However, if you perturb all atomic positions a little bit (there is a button in the Builder for that), then the space group is reduced to P1 also here.
