Edit by @unknown
The hitbox for wall banners is incorrect; it stands out someway from the wall it is hanging from. (See screenshot)
Original Description:
When you click a banner on a 1 block high wall it goes in the bottom half goes to the bottom block. The outline of the banner is also not placed correctly.
Simplifying this ticket to the most prominent of the multiple bugs posted here. It is much easier for us to deal with bug reports when they contain only one bug.
The issue you mentioned with banners going into the ground is working as intended, as it behaves that way on Java (see ticket MC-62987 for reference).
I have updated the ticket description and title.