One third party watch face claims to have heart rate working. I emailed the developer to ask what he had done to make it work. He did not reply. As it happens, the thing is open source. On inspection, I see that he bodged it, using Samsung Health APIs directly. I haven't found any other way to make it work, although Watch Face Studio works a bit, with the text tag [HR]. This updates very rarely. Hope Samsung fix this soon.
06-09-202108:56 PM - last edited 06-09-202108:57 PM
I've emailed a couple of developers and as I have quite a few thema designs for my Huawei watches I asked if will be updating to WearOS and work with Watch 4. They let me know they are updating and can now use complications that point to Samsung stuff, the only issue is that there are still some bugs that Samsung/Google need to fix for things to work properly. A lot of the older watch designs are also a lot better than ones being ported from Tizen as they aren't locked down and work when touch the complications. Most of the ones coming from Tizen developers don't allow you touch complications to get further info.