Ciao,
I have a problem with Runtastic app... near every time I use it on my Apple Watch with WOS 3, in 10-15 minute it crashes, and when I reopen the app, the heart rate is gone!
I find there are a lot of buggy apps on the watch. It seems as though third parties still don't really care about watch apps. Must be because the screen is too small to shove ads down our throats and it draws views away from the ads in the free app (I am looking at you weather network)