Based on your description, it seems that the issue with your 2016 570 EFI could be related to the starter motor or the starter clutch mechanism. The starter motor is responsible for turning the engine over when you turn the ignition key, while the starter clutch is a one-way clutch that engages the engine when the starter motor turns.
If the starter motor isn't staying engaged with the engine, it could be a sign that the starter clutch is slipping, preventing the motor from turning the engine over. This can happen due to wear and tear on the clutch or due to a lack of lubrication, which can cause the clutch to wear out more quickly.
One thing you could try is to inspect the starter clutch and the related components, such as the starter gears, for any signs of wear or damage. If you notice any issues, it's best to replace the damaged parts with new ones. Additionally, you may want to lubricate the starter clutch mechanism to ensure it works smoothly and efficiently.