The bug

Sometimes randomly unable to place ladder while standing in the same block.

The cause

The issue is that ladders are solid, and the hitbox detection upon being placed accounts for when it's facing north (or when it's facing south?), regardless of what direction it's actually going to be facing when placed. (I did not look at the code, this assumption is just from lots of testing.)

This means not only can you wind up being unable to place a ladder due to occupying the space of the north- (or south?-) facing hitbox, but also being able to place a ladder when you're right up against the wall you're placing it on, due to not occupying the space of the north(/south?) hitbox.

Video demonstrating the issue.