Transition not easing out when un-hovering

I'm making a simple button with a CSS underline ease transition. All other ease transitions are working fine (hover, ease in, un-hover, ease out), but border-bottom will not ease out. When you quit hovering, it simply reverts back to normal without easing out.

Here is a code-pen with a quick button I made to illustrate the problem.

http://ift.tt/2uf6eRx

Here is my CSS:

.gbtn {
    background: #bba989;
    text-align: center;
    line-height: 150px;
    height: 150px;
    color: white;
    text-transform: uppercase;
    letter-spacing: 1px;
    font-size: 12px;
    display: inline-block;
    transition: all .25s ease;
    box-sizing: border-box;
    font-family: "montserrat", serif;
    padding: 0px 30px;
}

.gbtn:hover {
    background-color: #aa9470;
    border-bottom: 150px solid #242424;
}