The adjustments you implemented have caused the sidebar width to expand and occupy all available space, thereby pushing the main content onto a new line. Although it resembles a collapsed navbar, in reality, it functions as an oversized sidebar.

Regarding the solution:

In Bootstrap, when the container reaches a maximum width of 768px, use the "md" class prefix.

To address this issue within the sidebar container:

<aside class="col-12 col-sm-3 p-0 bg-dark flex-shrink-1">

You need to modify the "sm" prefix typically used for mobile devices to "md", ensuring that the sidebar remains visible once the width hits 768px.

<aside class="col-12 col-md-3 p-0 bg-dark flex-shrink-1">

To preserve the collapsible functionality of the navbar at this width, switch the class from "navbar-expand-sm" to the "md" prefix as well.

<nav class="navbar navbar-expand-md navbar-dark bg-dark align-items-start flex-md-column flex-row">

Note the adjustment made to switch the "flex-sm-column" class to "flex-md-column" in order to maintain the row-oriented flex-direction at this specific size and avoid any layout disruptions.

https://i.sstatic.net/yD7zc.png

Complete code:

<div class="container-fluid">
<div class="row wrapper min-vh-100 flex-column flex-sm-row">
    <aside class="col-12 col-md-3 p-0 bg-dark flex-shrink-1">
        <nav class="navbar navbar-expand-md navbar-dark bg-dark align-items-start flex-md-column flex-row">
            <a class="navbar-brand" href="#"><i class="fa fa-bullseye fa-fw"></i> Brand</a>
            <a href class="navbar-toggler" data-toggle="collapse" data-target=".sidebar">
                <span class="navbar-toggler-icon"></span>
            </a>
            <div class="collapse navbar-collapse sidebar">
                <ul class="flex-column navbar-nav w-100 justify-content-between">
                    <li class="nav-item">
                        <a class="nav-link pl-0" href="#"><i class="fa fa-heart-o fa-fw"></i> <span class="">Link</span></a>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link pl-0 dropdown-toggle text-nowrap" href="#m1" data-parent="#navbar1" data-toggle="collapse" data-target="#m1" aria-expanded="false">
                            <i class="fa fa-address-card-o fa-fw"></i> <span class=""> Link</span>
                        </a>
                        <div class="collapse" id="m1">
                            <ul class="flex-column nav">
                                <a class="nav-link px-0 text-truncate" href="#">Sub</a>
                                <a class="nav-link px-0 text-truncate" href="#">Sub longer</a>
                            </ul>
                        </div>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link pl-0" href="#"><i class="fa fa-book fa-fw"></i> <span class="">Link</span></a>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link pl-0" href="#"><i class="fa fa-heart fa-fw"></i> <span class="">Link</span></a>
                    </li>
                </ul>
            </div>
        </nav>
    </aside>
    <main class="col bg-faded py-3">
        <h2>Main</h2>
        <p>Sriracha biodiesel taxidermy organic post-ironic, Intelligentsia salvia mustache 90's code editing brunch. Butcher polaroid VHS art party, hashtag Brooklyn deep v PBR narwhal sustainable mixtape swag wolf squid tote bag. Tote bag cronut semiotics, raw denim deep v taxidermy messenger bag. Tofu YOLO Etsy, direct trade ethical Odd Future jean shorts paleo. Forage Shoreditch tousled aesthetic irony, street art organic Bushwick artisan cliche semiotics ugh synth chillwave meditation. Shabby chic lomo plaid vinyl chambray Vice. Vice sustainable cardigan, Williamsburg master cleanse hella DIY 90's blog.</p>
        <p>Ethical Kickstarter PBR asymmetrical lo-fi. Dreamcatcher street art Carles, stumptown gluten-free Kickstarter artisan Wes Anderson wolf pug. Godard sustainable you probably haven't heard of them, vegan farm-to-table Williamsburg slow-carb readymade disrupt deep v. Meggings seitan Wes Anderson semiotics, cliche American Apparel whatever. Helvetica cray plaid, vegan brunch Banksy leggings +1 direct trade. Wayfarers codeply PBR selfies. Banh mi McSweeney's Shoreditch selfies, forage fingerstache food truck occupy YOLO Pitchfork fixie iPhone fanny pack art party Portland.</p>
    </main>
</div>

To achieve the desired effect, you need to modify the classes on certain elements in your HTML code. Start by changing flex-sm-row to flex-md-row

<div class="row wrapper min-vh-100 flex-column flex-sm-row">

to

<div class="row wrapper min-vh-100 flex-column flex-md-row">

Next, switch col-sm-3 to col-md-3

<aside class="col-12 col-sm-3 p-0 bg-dark flex-shrink-1">

to

<aside class="col-12 col-md-3 p-0 bg-dark flex-shrink-1">

Finally, change navbar-expand-sm to navbar-expand-md and flex-sm-column to flex-md-column

<nav class="navbar navbar-expand-sm navbar-dark bg-dark align-items-start flex-sm-column flex-row">

to

<nav class="navbar navbar-expand-md navbar-dark bg-dark align-items-start flex-md-column flex-row">

The concept behind these changes is to replace all the sm classes with their respective md counterparts to take effect at 786px (in md view) rather than 576px (in sm view).

Check out the updated JSFiddle