Watch is measuring data for heart rate but not logging on watch

Make sure you’re not wearing your Apple Watch too loosely. This can affect the sensor in the back. Before a workout, check the band and tighten it. When the exercise is over, you can lose it up a bit. In addition to tightening your Apple Watch, you should also clean the sensor (and the back) so that it can give a better result.

I would suggest that you reset your watch. Press the crown and side button in untill the Apple logo appears. Also Check your Watch is updated to 5.2 and your iphone is running ios 12.2.