Without more info/details about your setup it's hard to speculate.
You can also use the SEARCH at the top of the Meraki Community and search by splash re-login to see if others have (had) similar issues and what resolution actions were suggested.
How far apart are your APs? Close enough for a device to roam from one AP to another? Or too far from AP to AP where they are dropped? Has a wireless site survey been done (which might shed some insight)?
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