Did the trainee not apply Toga before the end of the runway?
That´s what I suspect.
And this would not be a "technical problem", but mishandling of the aircraft. Airbus did issue several publications to clarify the use of the throttles and the TOGA logic if no full power is required.
It is was not a go-around, but a touch-and-go.
Is it yet clear, how much it really touched? It may have kissed the ground, but not enough to trigger the ground logic. Excessive speed was mentioned in the reports.
The FDR will tell.