have a cost reduction effect in play that applies to a card with a monocolored hybrid mana symbol in its cost (in my case Baral, Chief of Compliance reducing instant and sorcery spells by {1} and Spectral Procession

attempt to cast that card for the lower cost using said cost reduction, choosing the generic side of the mono colored symbol

cost reduction does not apply (see attached screenshot where indicating (this mana cost costs {0} less)