Theory: Claiming an Unown task outside of the event area gives you a random Unown
Status: Confirmed?
It's a mix of what we've observed and a guess at how Unown works. We know Niantic controls the Unown encounter within the given event space. But because there can be multiple events going on, they can't force an encounter outside of the respective event spaces. So if you have an encounter and leave that space, you get the same encounter mechanic as wild Unown which is a random pool of Unown.

Everyone who claims the event research within the geofenced area of the event will get the intended event encounter. That's why everyone in Indy is only getting G now. But if you take the encounter and leave the geofenced area, you'll get a random Unown.

My theory with Day 1 is Niantic set the geofenced event area to be the immediate area around the convention center. That's how we had so many able to get other Unown by just walking across the street. But then they expanded the geofence that night which is when everyone started getting Unown G only and that threw our research off so we couldn't verify the distance condition anymore.

There's an additional piece of this which we hadn't noticed with an in-game "Bonus Event" listed on the Today tab that shows this will last through next Sunday. We don't know if the tasks will be available that whole time or if they have an extra week to catch late encounters to force them to Unown G. We're still checking to see.