HRV decrease since few weeks

not really. spring fatigue maybe? 

II've never see that before

No visible health issue. That's why I'm suprised

Can you show us the DAILY values the week before and after the firmware upgrade?

Llast month. Where I can see install firmware date ?

It doesn't matter. There's no sudden decrease. Your body is tired

Ok, I'll check regurlarly. Very strange obviously

You are young (20ys +-a few years) , if i had to bet , did you train more since 15th febr or any extra stress in your life (school maybe?) or it can be a small infection (even if you had no symptoms) what decreased your hrv till 25th febr, but then it went up to ~100ms as i see on 2nd march already.

I'm 46 years old. Thanks you for your estimation ^^. On the contrary, I would say that I train less than before (Achilles tendonitis). If you see my first screenshot, you'll see the fall.