Accordion/Default State doesn't do anything

Yes, i am struggling with exact the same behavior. The last accordion item always stays open.

@surfkid:

What you can try is this:

Change here:
src/Element/AccordionItem.php
one line of code in the module for the accodion item from TRUE to FALSE like this (and make a cache rebuild afterwards).

public function getInfo() {
    $class = get_class($this);

    return [
      '#process' => [
        [$class, 'processGroup'],
      ],
      '#pre_render' => [
        [$class, 'preRenderGroup'],
      ],
      '#open' => FALSE,
      '#theme_wrappers' => ['field_group_accordion_item'],
    ];
  }

At least it seems. that this hack works for me.

Here's a patch I made which seems to resolve the issue.

Sorry that didn't work, try this one.

Thx for the patch. I have committed this to dev

(sorry that was unintentional)