On a possible workaround – just did this for TrainerRoad;-)
- From the browser on your iPad, select download activity, open the FIT file, then share with …
- Airdrop to your iPhone
- On the iPhone, select HealthFit – a cool app for Apple Fitness users anyway – and import the FIT file
- HealthFit will automatically put the activity incl. HR (and GPS where available) into Apple Health
Which also shows it is easy enough …