Perhaps future updates, the ECG in Samsung health opens up Samsung Health Monitor, the Blood Pressure in Samsung Health is not yet integrated to read from Samsung Health Monitor, Samsung Health Monitor took basic information about you from Samsung Health and the sync ability which is again, not fully integrated yet, Samsung is still rolling it out, if you uninstall Samsung Health and reinstall, your synced data is restored, uninstall Samsung Health Monitor and reinstall, sometimes you lose the data and sometimes it's there...
The agreement is to allow applications to read from each other, so wait for updates and you'll see things working...
One last thing, uninstall Samsung Health Monitor, ECG dissappear from Samsung Health, Blood Pressure stays because it's part of Samsung Health, not Samsung Health Monitor...
What I think will happen, Samsung Health gets major update and it starts reading from Samsung Health Monitor...