Closely relates to MC-241035.

The bug

The vignette texture treats darker pixels as more translucent instead of treating them as darker and using the alpha channel to track translucency.

Given historical changes to textures throughout the updates, such as banner patterns in 1.15 (see https://www.minecraft.net/en-us/article/minecraft-snapshot-19w39a), it appears that this practice is not entirely accepted, and is being phased out somewhat, with the vignette being one of few leftover cases.

How to reproduce

This can be easily seen by just extracting the texture file from the jar and noting it being white around the edges, and black near the center, the inverse of how it appears in-game. A resource pack to test this in a more sophisticated way may be created in future.

Expected results

The texture file would be black at the edges and transparent in the middle.

Actual results

The texture file is white at the edges and black in the middle.

How to fix

The vignette texture file should be made translucent accordingly (don't forget Programmer Art!) so it renders as translucent in-game, and this "darker is more transparent" rendering method deprecated from it.