The bug

As lecterns are implied to be made out of oak wood, it'd be expected that the sides of the base plate would use the bottom two pixels of the oak planks texture, just like the bottom of the lectern does. This, however, is not the case, and it instead uses the middle of the lectern's base plate top texture (the one with the red on it).

How to fix

A model that fixes this issue is attached - simply replace vanilla's lectern model file with this to completely fix this issue.