It happens to me too (Galaxy Watch 5 pro). Most times I have spikes in HR measurements until it locks and reads well (it lasts about 10 minutes)..
The photo above illustrates the kind of information I have. Completely useless.
The pulse is correct during normal use, but when you start a workout such as walking or hiking, the pulse goes up and no longer drops. In other words, it has doubled. A user has found out that this effect occurs when reaching the pulse zones and then doubles. When the user stops the workout, the pulse on the watch face is also wrong and therefore doubled. If you then start a new workout, the pulse is retrieved again and is correct again until a pulse zone is exceeded and then the game starts all over again.
I´ve noticed that it happens mostly when I practice exercises early in the morning. I know it seems odd... but for the same exercises it happens that when I wake up and practice at 6am it is always wrong in the first minutes (abouot 10 minutes) and then locks OK. when I do it later like 11am or in the afternoon it´s ok most times.
It seems that it is a software bug... I hope Samsung delivers it as soon as possible