As it is well known, apart from +1 and -1 parities for particles and antiparticles, for intrinsic parity also defined "by convention" parity -1 for some symmetrical particles which are own antripartices. Such are, for example, photon and neutral pion.
From the point of view of physical meaning that is strong equivalent to statement that photon and neutral pion are antiparticles without particles, which severely violates real symmetry of that particles by convention for intrinsic parity.
Then, (surprisingly) in calculations on particle reactions that already violated by convention P-symmetry appeared to be violated again (of course) and physicists are making from it a big deal.
Explain, if you please, why?