The treb respawned after a while, got taken down to 10% and then got repaired miraculously again to functional percentage.
Did not see it happen on Beno (next fight), might be because it was a different keep or because the necessary class was not present anymore.
Test on own realm palintone was conducted if we were missing something about the repair being possible after all with following results:
direct heal won't work
life transfer (cabalist) won't work
repair with sufficient wood in the backpack won't work
It was not possible to check Hibernian spells like mentalist HoT or animist AE heal as we're Albs. Maybe some spell fell through the cracks when making keep siege items unrepairable.