The promotion-system doesn't seem to like Units with a withdrawl rate of above 100%.
With Sidar I built some Scouts (masked them since some got Esus) upgraded them to Horseman, Promoted to Flanking 3 (85 % Withdrawl Rate thanks to homeland), Mutated them and upgraded the ones which got Light Promotion (-20% strength +10% withdrawl) to knights (for another 10% withdrawl) so they got past the magical 100% mark. (You cant promote Flanking if you whould pass 85% Withdrawl with them it seems but Promoting first, then elevating the withdrawl rate seems to work.)
When finished and reached 17 EP or above by Fighting, they didn't got the ability to promote. What might be the reason for that? HN, missing unitcombattype of Knights or the 105% Withdrawl? Seems like a bug either way. Or is that hardcoded to prevent them from advancing further?
On a related Question regarding those units:
Can they still be killed normally outside a city then (immune to first strikes since they got Flanking 3) or whould they behave like loki running away every time they lose a fight?