Calories burnt in weight training

I don't know, but they're bound to be ridiculously inaccurate either way. Unless you enter in the specific exercises you did, reps, sets, weights, break times etc there's no way MFP could make an accurate estimate of calories burnt per weights session.

You can log the amount of time for "strength training" under the Cardiovascular section to get an estimate for Calories burned. You can use the Strength Training section to log sets, reps, and weights, but that section does not account for the amount of time or Calories burned.

Add "Strength training (weight lifting, weight training)" to Cardiovascular to get estimated Calories burned added to your Diary. Please note that the Calories burned for Cardiovascular exercises provided by MFP are based on published metabolic equivalent of tasks (METs), which are estimates for a general population and may differ for you as an individual. If desired, add individual strength training exercises, such as "Biceps Curl," to Strength Training to have a log of sets, reps, and weights as individual exercises.

Please see these articles in the list of articles on this topic in the MFP Help pages...
myfitnesspal.desk.com/customer/portal/articles/11170-why-don-t-you-calculate-calories-burned-for-strength-training-
myfitnesspal.desk.com/customer/portal/topics/455842-exercise-diary-and-exercise-database/articles

The amount of calories is pretty small anyway so accuracy really isn't that important.
Not worth stressing about trying to be accurate for something that can't be measured anyway.