WMI Provider Host remains at 20-30% even after exiting Garmin Express app.

Which Windows version, as it doesn't happen on my Win10 system?

Then something is wrong in your system then, not happening on mine, do you update it regularly?

I have found a setting that allows Garmin Express to start when the system is started and another that keeps it running in the background.  I had both set to allow these actions.  I've now reset these and the high CPU usage of WMI Provider Host disappears after I quit the app.  This is now the same behaviour that others who have reported the problem in this forum have found. The high CPU usage of WMI Provider Host shouldn't happen when the app runs so this is clearly a fault.  However as it now only lasts during the time that I am using Garmin Express it is not the problem that it was.  A high CPU usage can point to some malware or virus on your system and it took some time for me to track the cause down to Garmin Express.  So immediate problem resolved and I'll leave it to Garmin to fix the bug in their software.