Why does my 2019 MG3 unlock itself repeatedly?

This is another classic example of body-computer-fault behaviour. The body computer on a modern car controls multiple (hundreds, in fact) functions such as the heating and cooling, dashboard, media and entertainments, wipers, lights and, of course, the central locking. If the computer has a glitch, the central locking can develop a mind of its own.

Ultimately, you may need to replace the body computer, but this is potentially a warranty job since your car is a 2019 model and should, therefore, carry seven years of factory cover. Take it back to the dealership and have the situation assessed.