Does caching has to be manually disabled for ajaxified forms?
Not sure what you mean by "ajaxified". That sounds like a regular form is automatically "ajaxified".

Potentially this form is violating the principle that GET forms should not manipulate state. i.e. GET forms must be stateless. HTTP dictates this, but in Drupal 7 it was not respected. In Drupal 8 it is.

See https://www.drupal.org/node/2502785.