This isn’t how the Flex Pass works. You don’t need a new reservation to re-enter a park on the same day no matter how long you’ve been out of a park. So if you make a “Both Parks” reservation for a certain day, your AP does function like a park hopper ticket for the day.
The reservations switching to unavailable in the afternoon/evening would only affect a Flex Pass holder who never had a reservation for that day at all but decided that afternoon/evening that they wanted to go to the parks (in other words, a local).
A situation where you could be stuck in the Esplanade unable to re-enter would only occur if there was a capacity gate closure at the park you were trying to re-enter and would not be reservation related.
I don’t believe any “good to go” days have switched to reservation required thus far. This would constitute a change to the blockout calendar which, while something Disney has shown they’re willing to do, is usually accompanied with more announcement.
Oh that's fantastic to hear! Flex Pass it is then for me! For a 7 day visit it seems like a no duh option anyway and then hopefully I'll be able to get down there once or twice more in the next year. I was just worried about park hopping since I tend to do that A LOT each day (usually three or four times back and forth... I'm crazy)