It was same situation with me. Some replys suggests to use Group Policy, and sure I did.
But even clients with Group Policy still have same problem - users arbitrary get splash screen even it's in whitelist or group plicy with bypass splash.
And end of thread, it's because of Meraki's backend problem. But when I asked about it to Meraki TAC, they only says "It's not exact on which particular issue, case or bug identification number. What I can suggest is if to call us immediately when issue occurs so we can trace where is the issue is happening."
So, it's been 7 months after last reply. Does anybody knows about breaking MAC whitelisting and Group Policy situation's updates?