I have the same issue since v9.0.1.0. It may be related to switching specs when the original spec doesn't have anything bound to mouse 4/5. It doesn't appear to have anything to do with the class or spells, I'm able to reproduce the issue on druid/monk/pally. The "fix" I've been using is I open HB spells and click the dropdown for Mouse 4 and then Mouse 5. I don't need to set any of the binds again, just opening Spells > Button > Button4, and then repeating for Button5 makes the mouse buttons work again.
To give a few examples:
-Load into game as mistweaver with mb4/5 bound to spells. Everything works perfect
-Load into game as brewmaster with no binds for mb4/5. When switching specs to MW mb4/5 don't work until I open the healbot spells for mb4/5.
-Load into game as brewmaster with binds for mb4/5. BM binds work and when switching to MW they work as well.
My best guess of what's happening is:
-something is suppose to happen on load, but is ignored when no keys are set for the mouse button.
-When switching specs that code is suppose to run again to check if the other spec needs those buttons, but currently isn't
-This is why opening the HB menu restores the mb4/5 functionality.
-Note reloading after switching specs will also restore the button functionality, likely because it's the equivalent of running the code when loading into the game, except it sees the buttons are bound for mb/4/5 so it works.
Hope this helps with troubleshooting
Thanks
