Recovery heart rate starts when finishing workout with Zones app on Apple Watch.
Please refer to Recovery Heart Rate
If recovery heart rate measurement does not begin, check the following:
In order to measure recovery heart rate, it is necessary to finish the training with the heart rate zone above the threshold zone.
Even for training that includes heart rate zones not less than threshold, if your zone is a below threshold at the end of workout, the measurement will not start.
If you want to measure recovery heart rate, please end your workout with your heart rate zone more than threshold.
Please check Recovery Heart Rate for measurement of recovery heart rate and threshold setting.
Settings of iPhone app is automatically transferred to the Watch app.
However, since it may be possible that it was not synchronized due to some troubles, such as after installation, please confirm with Watch application settings.
For confirmation, please refer to Settings of Apple Watch App.