I'm not sure why input labels don't appear for mobile grids. In the meantime, this code will add the labels back:
<script>
$(document).ready(function(){
$('.mobile_grid .numeric_input').before('<span class="grid_cell_text">A</span>');
$('.mobile_grid .numeric_input').after('<span class="grid_cell_text">Z</span>');
})
</script>
This CSS will give the labels a small margin that may be a visual improvement:
<style>
.mobile_grid span.grid_cell_text {
margin: 4px;
}
</style>