I have a fully booked Akoth, except for the passive (booked him before ascending). If I go to the tavern and select "Upgrade Skills", by default, the "Show only upgradable" is checked.

With that box checked, Akoth does not show up in the list of champs to upgrade. However, if I uncheck the box, he does show up.

I can post screenshots if necessary, though the above description I think should be enough for them to find this, if it's not already known.