It's not really a Garmin issue, because the barometric adjusted Garmin data run through Garmin training center is pretty spot on - consistent with my old HAC4 data for the same rides and with other people's Polars, etc.

The problem is putting the data into MB without checking the new adjust feature in MB (I don't know what the new button is but there is now some option to allow MB to track more closely with the barometric data somehow. I don't use it so I don't know what it's called. We have a teammate who would always log our training rides and then broadcast his MB data and it would be at least 15% higher - really depended on the ride but it was always WAY higher. When he checked the adjust button on MB it went a lot closer to the GTC damat, but still maybe 5% high.)