7.60 on Fenix 5X Plus, walking/hiking heart-rate off

Since I upgraded the Fenix a few days ago the walking heart-rate seems to be completely out of whack.

Today I walked (carrying a backpack w/ shopping home) 10 km, and even on the decent climb on the way home the watch claimed my heart-rate was in the mid 70's, when a manual assessment (watch/pulse counting) suggested a pulse in the low 100s (105-120).

Same the day before on the way home.

In running mode, or when I just amble about at  home it seems fine.

Any ideas what's going on there?