What is unusual ? Using their owners' garden instead of a neighbour's, in which case it sure is; or not covering it up when they have gone on the lawn, which in my experience is not rare but seems to be compulsory for cats.
Stopping cats fouling where undesirable is an age old problem, and what works with one cat may not work with the next. Since it is your cat, can you not put the dirt box in the corner of the garden and train it to use that location ? If it is already used to a dirt box it may catch on fairly quickly. Once it knows where to go, you may be able to dispense with the dirtbox.