There are already old reports concerning this issue but it is still a thing. (MC-147444)
I tested it in singleplayer 1.14.3 with mobGriefing = false
The sheeps eat the grass and the grass dont dissapear.
With this bug it is possible to create op wool-farms when the gamerule is set to false!
This issue was fixes in 1.8 #MC-5570, but at the moment broken again.
It also causes an inconsistency, for example endermans, which blocks do not pick up when the gamerule is disabled.