This form was missing from the server cache on Drupal 8
I fixed that prevent that drupal cached the page, this way, because the forms can't be cached:

// Deny any page caching on the current request.    
\Drupal::service('page_cache_kill_switch')-&gt;trigger();


EDIT:

Recently was commited Add #cache[max-age] to disable caching and bubble the max-age

When $build['#cache']['max-age'] is not set, permanent cacheability (Cache::PERMANENT) is assumed. 

To indicate that a render array is not cacheable at all, set $build['#cache']['max-age'] = 0 (i.e. zero seconds).

And to indicate that a render array is cacheable only for a limited amount of time, e.g. 5 minutes, set $build['#cache']['max-age'] = 300; // 5 * 60 = 300.