Text on signs does not change shade with the sign texture

This is a follow-up to a closed issue MC-167625. It's been closed due to the fact that the text color was identical, as opposed to the issue's description. The problem is that when different rotation of a sign (wall sign or standing) causes the texture to have a different brightness, the text brightness stays the same, therefore it makes the text hardly readable in some cases.

Attached is a picture of two oak signs placed side by side, one facing north (rotation 8) and one facing east (rotation 4) in daytime with a light source in the same distance from both. They have been dyed using white dye and the text has the same color value. However, for some reason the brightness of the signs' textures is different and the contrast with the text is different, making one of them harder to read.