Intensity minutes - during an activity vs normal walk

Guys,

I'm new to a Fenix with HR. Used to have Fenix 3 util now.

When I walked for more than 10 minutes with Fenix 3 I would get the corresponding intensity minutes. When I walk with Fenix 6, I get nothing. But if I start "Walk" activity I get the minutes. Can you explain please? Do I miss some setting? Should not I get intensity minutes also for walks without starting an activity?

Thank you!

Best,

Marek