After all my years of riding and racing....I had this exact same thing happen to my new 2018 YZ450F. Odd and puzzling scenario! Unfortunately, it’s just a 5mm bolt that holds on to about three threads of the aluminum swing arm. I first attempted to just clean the threads up and replace with OEM slider with some blue loctite. After a couple of motos I heard the chain slap and realized that the bolt was once again gone. This led me to tapping to a 6mm again using a OEM bolt from front fork guard where yet again the same thing happened. Now, on chain slider two and frustration of mid to end day moto bike failures I chose to get more aggressive. I installed an aluminum nut setter which is similar to a pop rivet. I installed this nut setter and used a collared aluminum bolt with loctite applied again. I’ve now ridden several motos with success and per my identification markings on the bolt/slider hasn’t budged. Looks factory and will be simple to change chain slider if/when needs replaced or upgraded. ??