In materials explorer, for space groups #200-206, Pm3 should be Pm-3, Pn3 should be Pn-3…
We use the pymatgen.symmetry.analyzer.SpacegroupAnalyzer to retrieve space group data, including international space group symbol, which in turn uses the spglib library. To report this as a bug in pymatgen, please create an issue at the pymatgen code repository.
You may use the following code in your pymatgen issue to reproduce the situation:
from pymatgen import MPRester
from pymatgen.symmetry.analyzer import SpacegroupAnalyzer
m = MPRester()
s = m.get_structure_by_material_id('mp-6533')
sga = SpacegroupAnalyzer(s)
sga.get_space_group_symbol()
'Pm3'