If suddenly your power changes dramatically on your watch or iPhone while running at the same pace, or power is off when you upload your data with the Stryd app on your phone, likely the weight setting on your Stryd changed.
One example of how this can happen is when Stryd is never paired using the Stryd app on the phone. In that case, Stryd will keep its default weight setting of 70 kg. If you later on pair with Stryd, for example, to update the firmware, the weight will reset to match the weight in your profile which may be different than the default.
To address this problem, you can update your height and weight on your Stryd with the Stryd app. It is recommend to actually change the values when updating your height and weight (do it twice to go back to the original value).
Note that it is currently not possible to recalculate power of runs with incorrect data.
If you need further assistance, contact firstname.lastname@example.org.