I sometimes get the issue when you paint the grass then higher / lower the terrain, but not always.
PC - Windows 7 64bit, Intel i5, 12Gb Ram, 1TB WD Black, Evga Geforce 760gtx 2GB
Laptop - Windows 8.1 64 Bit, Core i7-3632QM processor, 8GB of RAM, 1Tb Hdd, Nvidia Geforce GT 740m