Heart Rate too low

Hello, I'm switching from a Vivoactive HR to the new Vivoctive 4.

After I few days of testing I found that the heart rate y too low. Vivoctive 4 is giving me about 50 bpm while walking and about 88 climbing a 15%-1km mountain with the bike (broadcast mode) which is impossible to me and totally different of what my chest strap says. With my old Vivoctive the HR was okay. Is anybody else having this issue?

Thanks!  Muscle