I have a map with "multiple boards" selected, and arranged to display a "Top" and "Bottom" board.
On both boards I have defined different rectangular grids, with "show grid" turned on.
The zone grid defined in the "Bottom" board is picked up at run time, but as soon as you save and reload the module the "Bottom" board reverts to showing the grid from the "Top" board. You need again to enter the grid editor and click "edit grid" to show it properly. But then you save, close and restart and it's back showing the wrong grid.
I backtracked and found that this bug appears from version 3.4.10.