Did a test game with a friend where we wanted to check which wargrider units are strongest.
Turns out RNG is a big factor in this but both wargrider units restored/replenished fighters in their ranks while engaged in combat.
We put two lv 2 units against each other and Isengards riders occasionally even restored two riders (not at the same time but one after another during the fight) without the whole batallion leveling up.
Not sure if this is a standard mechanic with other units aswell or just me not knowing that units actually "heal" in such a way during combat.
Units do not gain any HP or so, they just simply respawned a fighter as if they were waiting out of combat after lv 2.
I am repeating myself but that should make the potential "problem" quite clear.