When you stand on Lectern and press the sneak button and go to another corner, you can walk on another block site.
And your location changes when you stop at the northern right corner, as shown by the two videos below
MCPE-171101
The Lectern square is invalid
Resolved
Invalid
0
In the videos it looks like the edge of your collision box is at the edge of the lectern collision box. The coordinates show the position of the center of your feet, which is in the next block over. This is just how sneaking to the edge of a block works. There is nothing “invalid” about the lectern and, indeed, nothing specific to the lectern in what you describe.