I have what seems to be an intermittent problem.  It comes and goes away.  Right now I have it and ruins the game.

For no reason, an army outside a home district has it's health degrade.  Not from battle, in fact it can be completely healthy and start degrading.  If I can't race back to a home district in time, the army including the hero dies.

Any clues or help?

It's a winter effect.

Necro-ing for a fuller explanation

You're getting -15% Health Regen anytime you leave your or an Ally's territory in winter, due to a Pearl vote that likely the AIs did without you (Allayi in game loves it)

Sadly it calculates starting at Zero and gives you -15% and HP loss, rather than subtracting from your actual Health Regen (which could be at 60% for example and is now -15%, instead of say 45%)

Stay home or with an Ally during those winters and you will get +15% HP Regen in your territory.

This effect makes it MUCH easier to defend at home from aggressive factions, you heal more, they take constant damage, win/win.