The recently introduced changes to the field guide screen added the darkened background effect of the regular inventory when the field guide GUI is open. However, this effect is not properly scaled for higher field guide GUI size settings, as can be seen in the following screenshots, depicting the Large, Huge, and How large is your monitor even? field guide GUI scales at Minecraft GUI scale 3:

This can even happen on the Medium and Medium-Large field guide GUI scale settings, if the Minecraft GUI scale is set to 1

(For reproduction purposes, note that Patchouli only allows selecting larger GUI scales when playing with sufficiently large window size)