Currently, if Blood Moon (and probably Spreading Seas, though I have not tested it) is cast and there is no other reason for priority to be passed to you, it will resolve with no opportunity to float mana from nonbasics that would become mountains.

Situation that caused this: I have a Fragment Reality in hand and a single untapped Hallowed Fountain in play and there are no nonland permanents in play. My opponent casts Blood Moon, it resolves instantly and I have no opportunity to float white to cast the fragment reality on the blood moon.

Desired outcome: Without having to set a stop on my opponent's main phase (since that would be tedious, I do not know when they will cast blood moon), Arena should pass priority back to me to give me an opportunity to float mana from nonbasic lands whenever Blood Moon is about to resolve and there is an untapped nonbasic land in play.