There are two types of environment block in your layout page, as you can see below, Most of times we add the css and js in include development environment but misses the exclude development environment.
<environment include="Development">
<link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.css" />
<link rel="stylesheet" href="~/css/site.css" />
<link rel="stylesheet" href="~/lib/bootstrap/dist/css/CParts2018.css" />
<script src="~/lib/bootstrap/dist/js/CParts2018.js"></script>
</environment>
<environment exclude="Development">
<link rel="stylesheet" href="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.7/css/bootstrap.min.css"
asp-fallback-href="~/lib/bootstrap/dist/css/bootstrap.min.css"
asp-fallback-test-class="sr-only" asp-fallback-test-property="position" asp-fallback-test-value="absolute" />
<link rel="stylesheet" href="~/css/site.min.css" asp-append-version="true" />
<link rel="stylesheet" href="~/lib/bootstrap/dist/css/CParts2018.css" />
</environment>