Like Raywood, I'm not convinced the problem is only down to the wrong towball.
It is certainly true that the Alko caravan hitch should be used with an Alko style ball, but the reason for the "Special " ball is to allow the hitch the required full range of articulation. The Alko hitch uses the standard EU 50mm diameter ball, the main difference is the diameter and length of the stem that supports the ball. There should be no problem if a hitch is not taken to the limits of articulation and under normal driving conditions that could well be the case, but of course that might beyond the ability of a driver to avoid.
It is of course a concern that DW1311 has experienced a dislocation, and something of a mystery that it waited until it was on a fast road, which is unlikely to have twists and bends that might have over articulated the hitch and loosened it.
Its far more likely either the hitch was not properly engaged before the journey started or at some point during the journey the hitch was over articulated which has loosened it.
Because large articulations would be done at low speed, the hitch may have remained in contact with the ball, but as speed increased on faster roads the wind pressure on the front of the caravan may have been enough to reduce the hitches downward pressure enough to allow it to dislocate.
I note the OP's partner has HGV experience, and whilst I would expect such a person would be perhaps more aware of the need for care when hitching, but ball hitches are rarely used in commercial HGV trailers, so it might have been a new experience, and thus may not have fully understood the process.
I also note the incident involved a pick up truck. This raises the possibility the tow ball height may not have been within the 350 to 420mm height range, and the vibrations through the vehicles chassis may have had some adverse affect on the security of the hitch.
This is of course all speculation.