We have noticed something similar across a dozen or so MX67 devices - CPU will remain constantly > 80% even during low/no traffic periods (ie. overnight). Running a mixture of 18.211.2/3/4
Meraki support have said:
"Following an investigation of backend logs, we have confirmed that the MXs are being affected by an unexpected behaviour which causes the device to run at 100% CPU usage when one of the LAN ports is converted to WAN2, IDP/IDS is enabled, and a WAN change is made.
The behaviour was already raised to our development team who are actively investigating and looking to produce a fix as soon as possible.
Unfortunately, there is no ETA for when the fix is going to become available, at this moment in time.
The workaround to reduce the CPU usage is to either restart the MX or disable IDP/IPS for around 5 minutes and re-enable it."
We are in the process of scheduling reboots for these devices to see if it fixes it.