Form textfield AJAX validation "deletes" injected services to form
For anyone running into this problem or AJAX call that fails on the second call intermittently, check the properties used on your AJAX form class for the right access. It seems a trivial thing but change the properties of DInjected services to protected. Refrain from using private as the dependency serialization from Drupal won't be able to get those for caching purposes.
Check this link for more information.