Hi ,   https://documentation.meraki.com/General_Administration/Monitoring_and_Reporting/Device_Uptime Requirements and Limitations Device uptime is supported on all Cisco Meraki switch models and most access points. Support for other products is planned for future availability. ... View more

You need to raise a case with Meraki Support.   My guess is that the standby MX is trying and failing to upgrade its firmware - or something along those lines.   Support should be able to dig further.  Suggest calling on the phone - it'll likely save a bunch of time. ... View more