Fenix 7x Pro Solar, intensity minutes?

Hello, 

I own a Fenix 7x Pro Solar.

So far it has realistically counted the intensity minutes. 

Recently, it counts only very very few intensity minutes.

Previously, I had the 150 minutes already full on a MTB tour.

Now I get with 4 tours just 23 minutes together!

Although according to records, the pulse has already been over 100 for 90 minutes on only one tour.

Does anyone have an idea what this could be?