I had problems when I synced my Garmin with MFP. One day, I exercised for about 80 minutes total (hard exercise but only 80 minutes), and then went about my normal day. Garmin had me at burning 4500 calories that day! I realized the problem was it was adding 1500 baseline calories from MFP to the total burn Garmin had recorded. I unlinked the accounts and everything went back to normal. MFP worked great with my FitBit, but it seems to have issues with Garmins.

*I have a Forerunner 235.