Unexpected imaginary frequencies during the SCPH calculation

[Lamed-git]

Dear Dr. Tadano and users,
When I calculated a tetragonal crystal system, if I use a supercell (2 2 1, 64 atoms) derived from the conventional cell to fit the force constants, the harmonic phonon spectrum shows no imaginary frequencies. However, when performing SCPH calculations using the primitive cell, the calculation fails, indicating a lack of consistency between the supercell and primitive cell. Therefore, I switched to using a supercell (2 2 2, 64 atoms) derived from the primitive cell to fit the force constants. However, two issues arose: firstly, imaginary frequencies appeared near the Gamma point in the harmonic phonon spectrum.

I attempted to resolve this by using a larger supercell (3 3 3, 216 atoms) to no avail. Hence, I reduced the cutoff value for the second-order force constants from None to 18 Bohr (considering that this might be due to insufficient description of long-range interactions), which eliminated the imaginary frequencies. I wonder if this operation is reasonable.
Secondly, however, when considering the Bubble process in the SCPH calculation (supercell of 216 atoms, 18 Bohr cutoff for harmonic force constants), imaginary frequencies also appear at the Gamma point in the renormalized phonon spectrum. I'm not entirely sure about the source of these imaginary frequencies and how to eliminate them.