Other Workout App not showing accurate calories burned

A few weeks ago i switched from an iPhone 6 Plus and an iPhone 6S Plus. Ever since then I've had issues with the Other Workout App. When I had my iPhone 6 it would show accurate results, but now it's showing an extremely low calorie burn for the exact same workout

For example, I did an intense cardio workout with an average heart rate of 170 BPM. These are the results.

It only shows 168 active calories burned and 219 total calories. It used to show around 450 which is a more accurate reading according to most heart rate/calories burned calculator. Furthermore, I did a test and turned on the Other Workout app AFTER my workout for the same amount of time. It read the following:

My resting heart rate was pretty high because it was right after the workout, but nonetheless according to my Apple Watch I burn only 30 more calories during an intense cardio workout than I do not doing much of anything.

Here's an example of what the watch used to read before I switched phones:

436 active calories on a workout that's only 5 minutes longer with a lower average heart rate. This seems like a much more accurate reading and obviously a HUGE difference than what it is showing now.

For the record I've tried resetting my watch, unpairing and re pairing and re calibrating with a 20 minute outdoor walk with no success. I appreciate any assistance.