In fact, the decay is not forbidden, even though it is a rare decay as you could check on PDG. If you're interested you can check this (and many others) article https://arxiv.org/pdf/0901.4854.pdf

In fact, the decay is not forbidden, even though it is a rare decay as you could check on PDG. If you're interested you can check this (and many others) article:

• A. V. Anisovich et al., "The $ρ → γπ$ and $ω → γπ$ decays in quark-model approach and estimation of coupling for pion emission by quark", Phys. Atom. Nuclei 73, 462–477 (2010), arXiv:0901.4854.