The problem is that ACF is expecting the post_type argument to be an array at this point in the code and you are setting it to a string in your filter.
As a work-a-round use
'post_type' => array('post'),
the filter you are using happens after ACF has formatted the arguments and before it formats the values into json.
I suspect that this needs to be adjusted for PHP8 to see if the post_type argument is a string or not before line 261. But I can’t fix that. Contact the developers here.
