Elevation totally erroneous

Hi.  I have pretty new Edge 840 Solar.  It’s recently started to report elevation data that is totally unrelated to the ride.  I’ve raised a support ticket with Garmin Australia after waiting over an hour on the phone.  That was over a week ago.  They are not however not coming back to me nor responding to any follow up mails and their call center takes over an hour to get through to so is not an option.

Has anyone else had this issue and have a fix? Any idea how to fix? The unit is still under warranty but the customer support from Garmin here seems to be non-existent.

I’ve recalibrated the altimeter and reset the unit - issue persists. I’ve attached some elevation profiles of the exact same ride.  The second one is what the 840 is producing now.  The profile is totally out of whack with the actual route profile - recorded on the same device a few months prior.

Thanks for any advice to resolve this.