I'm not so sure it is a bug that you have more skills by default for a more advanced unit, but I forwarded this to the right person anyway
Nice try.
The right person will say "bah" and then fix the bug.
And it is a bug because it works contrary to your central game design and we are not talking about upgrading to a more advanced unit but about the same unit being guided along a progression path.
You reward the player for keeping his units alive and using them throughout the campaign. This reward takes the form of greater combat abilities as well as an overall bonus based on not losing units. When you advance in tier you sometimes get new units available, but you are always able to choose your old units (of the baseline types) into these new units keeping their experience and the abilities you have chosen, losing your own base-type
spells and gaining the base
spells of the new type, and it is always better to do this than to disband your old units and recruit level 1 units of the new type.
In other words, nursing units through the campaign is unambiguously good.
Elmet Legionaries are the sole exception I have found. If you hire them in T2, you are a fool if you keep using them in T3+. Unlike every single other recruitable unit in the game, there is a point in the game (T3+) in which you are better off disbanding them
and then immediately rehiring a new level 3 unit of exactly the same type as the one you disbanded, because the Elmet Legionaire you disbanded, who is every bit as "advanced" in the tier sense as the one you recruited to replace him, lacked the fomorislayer ability.
But you have a point there that it seems incorrect that they don't get it when they become T3.
I sure do.
They should either have it in T2 or gain it when advancing T2->T3 by swapping its old
spells for the new ones upon tier change. I don't care either way, but I hate seeing such inconsistencies that do hurt, even if only slightly as in this case, core player encouragement mechanics.