I had the same problem with my IDevices on my home network. On a regular use, with a 5 GHZ SSID with WPA2 PSK validation, session dropped when the iDevice was idle.
Jun 18 08:20:56 MR33 IPA iPhone Dal 802.11 disassociation client has left AP
Jun 18 08:20:56 MR33 IPA iPhone Dal WPA deauthentication radio: 0, vap: 0, client_mac: 6C:72:E7:9B:91:66 more »
Jun 18 07:52:08 MR33 IPA iPhone Dal WPA authentication
Jun 18 07:52:08 MR33 IPA iPhone Dal 802.11 association channel: 11, rssi: 28
Jun 17 19:25:22 MR33 IPA iPad Jorge 802.11 disassociation client has left AP
Jun 17 18:11:40 MR33 IPA iPad Jorge WPA authentication
Jun 17 18:11:40 MR33 IPA iPad Jorge 802.11 association channel: 11, rssi: 23
I tried most tips on Meraki MR troubleshooting. It got somehow fixed pushing (and forcing) a custom WIFI profile via System Manager, enabling Cisco Fast Lane and Sentry validation, but that solution doesn't apply to you problem.
I recommend opening a case in Meraki help center.