Leash burstes when placing a block on the fence where a horse is tied to.

Hello Team of Mojang,

when I tie a horse to a fernce and i place a block upon this fence the leash bursts and the horse is free.
But when I tie a horse to a fence where's already a block upon it it works.
Sorry for my bad English!

Yours sincerely,
Sven Hutzler