Em um projeto recente criado com o Bootstrap 4 , tentei todos os métodos acima, mas nada funcionou. Minha abordagem foi editando a biblioteca CSS usando o jQuery para obter 100% da tabela.
// * Select2 4.0.7
$('.select2-multiple').select2({
// theme: 'bootstrap4', //Doesn't work
// width:'100%', //Doesn't work
width: 'resolve'
});
//The Fix
$('.select2-w-100').parent().find('span')
.removeClass('select2-container')
.css("width", "100%")
.css("flex-grow", "1")
.css("box-sizing", "border-box")
.css("display", "inline-block")
.css("margin", "0")
.css("position", "relative")
.css("vertical-align", "middle")
Demonstração de trabalho
$('.select2-multiple').select2({
// theme: 'bootstrap4', //Doesn't work
// width:'100%',//Doens't work
width: 'resolve'
});
//Fix the above style width:100%
$('.select2-w-100').parent().find('span')
.removeClass('select2-container')
.css("width", "100%")
.css("flex-grow", "1")
.css("box-sizing", "border-box")
.css("display", "inline-block")
.css("margin", "0")
.css("position", "relative")
.css("vertical-align", "middle")
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet"/>
<link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.7/css/select2.min.css" rel="stylesheet" />
<div class="table-responsive">
<table class="table">
<thead>
<tr>
<th scope="col" class="w-50">#</th>
<th scope="col" class="w-50">Trade Zones</th>
</tr>
</thead>
<tbody>
<tr>
<td>
1
</td>
<td>
<select class="form-control select2-multiple select2-w-100" name="sellingFees[]"
multiple="multiple">
<option value="1">One</option>
<option value="1">Two</option>
<option value="1">Three</option>
<option value="1">Okay</option>
</select>
</td>
</tr>
</tbody>
</table>
</div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.7/js/select2.min.js"></script>