the space group of Mg2Ni has changed when structural relaxation

[joyouszy]

Dear everyone,
I am sorry that I have to ask a favour of you, with the hope that you will be kind to help me.
I am a newer to use VASP code. Recently, I encountered a strange phenomenon when I used VASP to optimize the structure of Mg2Ni, which belongs to hexagonal system. With the improvement of convergence criteria, the space group of the Mg2Ni changed from P6222 (SG Number: 180 ) to C222 (SG Number: 21 ). What can I do to preserve the crystal structure of the Mg2Ni from change while the energy and force convergences are 1e-8 eV and 1e-6 ?/eV? Is there anything that can be done about the problem? Please tell me detail as much as possible.

Encl: INCAR and KPOINTS
> Automatic mesh
> 0
> Monkhorst Pack
> 8 8 8
> 0 0 0
> INCAR
> SYSTEM = Mg2Ni-180
> PREC = Accurate
> ENCUT = 350.000
> IBRION = 1
> NSW = 150
> ISIF = 3
> EDIFF = 1.0E-08
> EDIFFG = 1.0E-06
> ISMEAR = 0
> SIGMA = 0.05
> IALGO = 38
> LREAL = .FALSE.
> ADDGRID = .TRUE.

[support_vasp]

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