How to adjust Garmin (incorrectly) calculated exercise calories

Sometimes my Garmin watch heart sensor plays up (I'm assuming too much sweat has crystalised on the sensor, or something like that), and the heart rate data comes in way lower than it should be. This, naturally, significantly reduces the amount of calories that it says I have burned.

How do I correct this in MFP?

I can correct the figure in Garmin Connect to match that on previous walks, but MFP doesn't update the calories burned for that exercise.

If I update the calories burned for the exercise in MFP, then that just creates a copy of the walk with those calories, but the Garmin Connect Calorie Adjustment goes up by the same amount, leaving me with the same incorrect total calories burned calculation.

Obviously, I know that I can just eat 100 extra calories, if I choose to, but I do like my record to be correct!