Form not receiving a CSS class, it's block wrapper does instead
Obviously the block plugin thinks the #attributes key in the returned array is meant for the block wrapper. So I would try to put the form in a subkey, this should prevent that the block plugin can access the attributes of the form:

  return ['form' =&gt; $this-&gt;formBuilder-&gt;getForm(...)];


By the way the core search block is also returning the form array directly like you did in the question and indeed, the resulting form tag in the search block has no classes too. I'm not sure, but this seems to be the same issue.