Closely relates to MC-54988, which has recently been reopened.

The bug

The dark oval displayed around the screen in dark regions is only present in the Fancy and Fabulous! graphics settings.

According to this 2014 post on Twitter by Dinnerbone, the fast graphics option is only intended to change the transparency of leaves as well as rain particles, neither of which are relevant to the presence of this vignette.

How to reproduce

As this effect is rather subtle and hard to see, the following reproduction steps rely on MC-73730. If said bug is fixed, this may require a keen eye to reproduce.

1. Use Fancy/Fabulous! graphics
2. Enter a dark area
3. Press F1, and note how the screen ends up getting brighter due to this darkness being removed
4. Now switch to Fast
5. Switch F1 again, and notice no change

Expected results

In line with Dinnerbone's comment, this vignette would still be present even in Fast graphics. As such, F1 would result in the same visible difference it did in Fancy/Fabulous!.

Actual results

The effect is absent entirely such that F1 makes no difference.