It is possible that the left rear wheel bearing was damaged by chain maladjustment, resulting in excessive tension on the chain. If so, the chain will probably have pin-wear. Overloaded chains are also prone to cracking rollers. Either of these faults may result in the sprockets showing some signs of damage. I suggest that when you have the chain off, you hold it on edge horizontally and see if it tends to sag a bit in the middle. Also check for cracked or broken rollers. If it shows either of these faults, it needs to be replaced because it will be out of pitch, and the excessive pitch of the chain will cause the sprocket teeth to wear in a deformed way. If you post pictures of the sprocket teeth, from a point in line with the sprocket's axle so we can see the teeth clearly, we will be able to tell you whether they are worn.
