If you have a lot of hair on your wrist, it can mess with the heart rate sensor. Try to shave it down a bit to improve accuracy.

For those who come after.

Let me guess. It’s a rebranded Chinese phone that typically goes for like 50 bucks?

If you can still separate the pieces: Take some big enough flat surface (has to be very flat) and cover it with sandpaper. Sand the pieces until they are completely flat. They will then connect without any gap in between.