Update: We have identified the issue or at least found where it resides. Turns out the problem is indeed related to high latency/packet loss somewhere in the AT&T MPLS network. We set up PCs at each end of the MPLS circuit, directly connected to the PE routers and configured with static IPs in the same network as the 2 switches. We used PRTG network monitoring software so that we could monitor the circuit without spanning-tree related interference. We had 2 root-bridge reelections again this afternoon. The event logs in the Meraki network showing this and the testing logs from PRTG align perfectly for both events, down to almost the second. We see errors/timeouts in ping/traceroute testing, and usually the first successful trace after the timeouts shows latency between 700-950ms in the monitoring event log for PRTG. So, it does not appear that the Cisco or Meraki devices are having any issues, and the Meraki is just doing what it's programmed to do when the root-bridge is no longer reachable. Thanks all for your help on this one.
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