Menu Button 3 No Longer Works to Show Overview Grid

Since updating to 2.8.12 under Mojave 10.14.6 the button assignment for the Overview Grid has stopped working. Specifically, using button 3 (which is a scroll wheel click on my mouse) now displays the grid for a fraction of a second then disappears. When displaying the grid from the menubar menu or using a hot corner, it still works fine.

Current workaround is to assign a different keystroke to it, and use the Mouse Button 3 in Keyboard Maestro to activate it.