If the DFT paper is correct, then annealing would let the Cu atoms relax to the Pb(2) sites, which doesn't induce potentially superconducting bands.
Metastable alloys (roughly, alloys that are only stable at room temperature because the atoms get frozen in place too quickly for them to reach the energetically favorable positions) are quite common in materials science, both for bulk metals and for semiconducting crystals. There are far more possible metastable alloys than stable alloys, so the range of material properties possible in metastable alloys is far wider than that in stable alloys. So, considering we have never discovered a room temperature superconductor, so far, it wouldn't be particularly surprising if it requires a metastable alloy.
Metastable alloys (roughly, alloys that are only stable at room temperature because the atoms get frozen in place too quickly for them to reach the energetically favorable positions) are quite common in materials science, both for bulk metals and for semiconducting crystals. There are far more possible metastable alloys than stable alloys, so the range of material properties possible in metastable alloys is far wider than that in stable alloys. So, considering we have never discovered a room temperature superconductor, so far, it wouldn't be particularly surprising if it requires a metastable alloy.