[This ticket has been merged into MC-268042.]

The bug

The texture used for the sides of lecterns contains regions of pixels which go completely unused by the lectern model. There are also some pixels that are used by the model, but are entirely obscured by other pieces of the lectern, and therefore never visible in practice unless the player clips inside of the block. As a result, these pixels can be removed from the texture file without at all affecting how the lectern looks, making it obvious for resource pack developers what parts of the texture are used and what parts can be ignored.

How to fix

An easy fix - just replace the vanilla lectern side texture file with the one attached.