Let $G$ be the group $GL(n,F)$, where $F$ is a p-adic field, and $I$ its standard Iwahori subgroup. Let $\pi$ be an irreducible smooth representation of $G$ with nonzero $I$-fixed vectors. It is well known from the Borel-Matsumoto theorem that such a representation embeds into an unramified principal series representation.
My question concerns the following unicity problem: Is it true that there is a unique, up to isomorphisms, irreducible smooth representation of $G$ (with trivial central character) with nonzero $I$-fixed vectors and with no nonzero $P$-fixed vectors, where $P$ is a fixed fixator of a panel of the affine building $X$ of $G$ (or any fixator because the fixators of the panels of $X$ are all conjugate).
The Steinberg representation of $G$ gives the existence of such a representation.
Is there any reference about this question ?