Are there any characteristics that determine a Pokemon's Hidden Power type in Pokemon Go, and if so, what are they?
In the mainstream Pokemon games, it was determined by the IVs. However, I suspect that either this is no longer the case, or other factors are in play. A Solosis I had caught had Hidden Power Poison on capture, but it learnt Hidden Power Dark when it evolved into Reuniclus. The IVs were 15 Atk / 12 Def / 12 Sta in both forms, and I did not trade it to reset the IVs.