FORM_FIELD_... process instance variable is missing in engine 7.15

For each Form Key definition of a UserTask in the BPMN, in all Camunda engine versions before, each process instance was automatically provided with a variable FORM_FIELD_… where the form key definition was stored as Json.

Under 7.15 this variable is missing now.
Is this a bug or has something changed in the logic of generating the FORM_FIELD variable ?