Missing power data

It has no way to do so. GPS does not work on a treadmill, and there is no data connection from the treadmill to the watch. This can be easily confirmed by the fact that no run on a treadmill without a footpod or some other valid source of pace data has any pace data (I have such activities, with just heart rate, from the past).

The watch uses the accelerometer to calculate the pace on the treadmill. For the treadmill activity type the GPS is turned off.

"Your watch calculates distance for indoor activities based on the internal accelerometer data combined with physiological data and historical training data. To give the watch the best data, make sure that you are swinging your arms naturally while running or walking on a treadmill. It is important to not hold the handrails in order for the watch to record arm motion. The watch accelerometer is automatically calibrated by doing outdoor runs with GPS recording, but there can still be differences in what your watch records versus the distance reported by the treadmill."

Read more here: Using the Treadmill Activity Profile on My Garmin Watch

Sorry, but this is not relevant as my watch -  a Garmin Forerunner 245 - has never been used in this way (Thanks for drawing my attention to this, but I have never done the calibration the manual explains is needed to get rough estimates of treadmill distance).

While I have never had treadmill pace without using a Stryd footpod to measure,  by chance I have an additional proof that the data came from the Stryd. I was using a 5% slope, but prefer to record the equivalent pace on the flat (based on power output), so I was using a correction factor of around 1.3 in the Stryd calibration. Thus the pace recorded was clearly different from the actual pace I know the treadmill was running at. An independent watch estimate of pace would not be using the Stryd calibration factor, so would be a lot slower if it was not very inaccurate.

So, although my knowledge was incomplete, I can be sure the pace and distance data came from the Stryd, and the mystery of why the power data is missing remains unsolved.

6446252 said:

my watch -  a Garmin Forerunner 245

The documentation at Garmin Running Power Device Requirements does no list Forerunner 245 among the models supporting Running Power natively, so unless the article is wrong, I guess that you are using a Connect IQ app for running power. If it is that case, then I'd check whether you enabled the CIQ app in the Treadmill Run activity profile too, and I'd also verify whether the concerned CIQ app supports treadmill runs at all.