I had bald spots when I first started the game and others appeared, not really connected to my path system.
I don't understand them or why they appear: some of the early research into grass wear suggested that some areas were more prone to wear than others; it was never more than a suggestion and was never looked into.
Yes you can grow the grass back in these spots, but I've only been successful if I plant flowers.
As they tended to be away from my path system, I use gold roses (you don't have to water).
Or use ordinary spare flowers, don't water and just replace when they disappear.