Does rage generation stop after twentysome turns?

[Petwin]

I made a mana/rage machine:
- Let Royal Thorns summon thorns.
- Dig up altars and gather the thorns around the altars.
- The altars damage the thorns, which generates rage.
- Summon more thorns to use the dead bodies of the fallen thorns to make more thorns.
- Use rage to cast mana accelerator with the pet dragon.
- End fight with max mana and rage.

After full mana restore I let the pet dragon lay mystic eggs to gather experience and create more bodies to be damaged.

But I normally don't get to restore all of my mana because at some point damage doesn't generate rage anymore. Is this a build in feature?