How to make opened accordion header sticky in bootstrap 4?

Have a BS4 accordion, want to keep opened header on top when users scroll down, so sticky behaviour for accordion opened header

actual approach is not working:

#accordion .card-header BUTTON:not(.collapsed) {
    position: -webkit-sticky; /* Safari */
    position: sticky;
    top: 0;
}

The header keep to go over the screen not visible

The code:

<style>
#accordion .card-header BUTTON:not(.collapsed) {
    position: -webkit-sticky; /* Safari */
    position: sticky;
    top: 0;
}
</style>

<div id="accordion" class="groups">
    <div class="card">
        <div class="card-header d-block d-md-none m-0 p-0">
            <h2 class="mb-0">
                <button
                    class="btn btn-link btn-block p-2"
                    data-toggle="collapse"
                    data-target="'#group-1"
                    aria-controls="'#group-1"
                    >title</button>
            </h2>
        </div>

        <div id="group-1" class="collapse" aria-labelledby="headingOne" data-parent="#accordion">
            ...
        </div>
    </div>
</div>