Bootstrap 4 Collapse Accordion - always have one panel open

You can use the collapse hidden event. In this case, use .eq(0) since 0 is the index of the first collapsible div.

$('.collapse').on('hidden.bs.collapse', function () {
    $('.collapse').eq(0).collapse('show');
})

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To take it a step further, you could add a new default data variable to the parent #accordion...

<div id="accordion" class="accordion" data-default="1">..</div>

And, then change the jQuery to use that variable..

/* default accordion variable method */
$('.collapse').on('hidden.bs.collapse', function () {
  var defaultDiv = $($(this).data("parent")).data("default");
  $('.collapse').eq(defaultDiv-1).collapse('show');
})

Demo: https://www.codeply.com/go/NilPIQD9oi

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Another option is to prevent any open accordion from closing itself, as I explained in this answer.