Try to use the important next to height: (see below)
@media (max-width: 991px)
.header .search-input-container input.search-field {
border-color: transparent!important;
padding-left: 15px;
background-color: #f4f4f4;
border-radius: 3px;
height: 40px!important;
transition: .2s background-color;