Apple Watch not showing as data source for HRV

The day I switched from Watch 3 to Watch 4 HRV data was no longer recorded as reflected in the Health app. Watch 4 also does not show up as a data source for HRV readings (and the old watch is still listed there). It is, however, listed as a source for other readings (e.g. heart rate, VO2max etc) and those readings are taken without any problems. I have used the Breathe app on the watch to try and force a HRV reading but it only records normal heart rate. I have an iPhone 8 and all the latest software updates on the phone and watch. I have tried repairing the watch and phone but no change. Any ideas?