Sent the bug report via game on this as well, but not sure if they always get delivered properly, so posting this here as well, now with extra images:
So apparently if upon meeting Woljif again in Act 3 you try levelling him via ‘Level Up’ button from the Interface screen, the button stays after each level and is actually clickable so you can continue levelling him!
Not sure if this is indefinite as I had enough after level 31. If you go past level 20, the interface breaks afterwards, as upon clicking on Woljif it still keeps displaying whichever character you had before, merged with some of the Woljif’s values - possibly connected to the game not being able to process spellbooks once the caster level is over 20 - which means you cannot then memorise and use any of your new spells.
However, in-game all the other abilities, feats and other features still work as if Woljif’s successfully levelled up to whatever.
And if he’s only levelled up to 20, everything (including spellbooks) works as it should, so if anyone feels like playing beta with an overpowered character on standby to breeze through some of the harder fights without the need of any mods, now’s your chance
P.S. I’ve tested this with levelling him via clicking the plus sign from his portrait directly, and that way only levels once. I’ve also tried clicking the persistent ‘Level Up’ button with some other characters when levelling them up through the Inventory screen, but it doesn’t do anything. So not sure whether there are any more occasions when you can indefinitely level someone up like that, but so far it seems to be exclusively Woljif, and only in that particular way (i.e. clicking from Inventory interface).