Regarding open world Heavy

[MurakumoMillennium]

Heavy is mostly inflicted by high level monsters (the ones near Primal grounds) or the inconspicuous ones (such as the Overgrown Offerings in South Shroud).

Heavy is inflicted by every mob in the open world capable of hitting you from behind while you're mounted. That includes lvl 1 squirrels and lady bugs.

Monster-inflicted Heavy is not subject to Diminishing Returns. This means that every time Heavy procs, you'll be slowed for 20 more seconds

That's because the "heavy" effect inflicted by being hit from behind is different from the casted spell version. The spell version of monster heavy is subject to diminishing returns.

Most, if not all Heavy inflicting attacks are ranged

That's because most, if not all, of the attacks that hit you are ranged. They can't melee you from a distance.

But yeah, I agree it's annoying when you can't just charge into a beastman stronghold solo past 50 mobs while being impervious to any and all of their attacks. They shouldn't even fight back because we can just kill them so easily!