I use bootstrap v4.3.1 and and select2-bootstrap4-them v1.2.2
The background of select is grey at place of white.
And it seem labels are not correctly aligned verticaly.
I had fixed the problem with those lines :
.select2 { background-color: #fff; }
.select2-container--bootstrap4 .select2-selection--single .select2-selection__rendered { line-height: 2.45em; }
.select2-container--bootstrap4 .select2-selection--multiple .select2-selection__choice { margin-top: 0.5em; }

Thanks for your job ;-)