[WaifuApple]

If you look in the bottom corner of the image, you'll see that the green box does not go all the away around the battlefield anymore. This is not just a visual effect - you cannot flee at all where the green border isn't present, and it seems to happen pretty much universally across battle maps from what I can see.

[AkaInuHime]

I discovered that it lets you flee from the middle of the map sometimes too.
Perhaps the green border isn't being sized to the full map properly, so it's like a smaller square outline of fleeable tiles?

[Neus]

This has been corrected in 2.84c.