That's not an answer! You can still do everything with 20% of HP so why shouldn't the AI also do it?
You will not solve this problem by restricting the AI. You need peace content (meaning less wars), perhaps lengthier sieges/heavier casualties to make the wars last longer (giving time to all other factions to replenish), etc. This is a design flaw, trying to solve it by patching it will only create more problems.
You could wander around with your army at 0% too and I think no one should be doing that.
I'm all for peace content but one reason why warband didn't feel like it had endless wars was because lords took time to heal and replenish. Now they're like roaches. There's literally no down time for them at all.
The minute they're released, they respawn at their base with some elite troops and bang..they're off recruiting. Before I know it, the army I destroyed is back to get destroyed again. Sometimes, I don't even have time to ransom the prisoners I have.
Don't tell me that's great game play. It's not.
And lengthier sieges/heavier casualties? I'm all for that. Heck. I wrote it a few posts above.