Scripts

Your code only runs at the start of the map. Therefore, you need to attach the trigger not only at the start but also when the hero appears:

function UpdateHeroSpecs(hero)
  if hero == 'Isolda' then
    Trigger(HERO_LEVELUP_TRIGGER, hero, 'IsoldaSpec')
  elseif hero == 'AnotherHero' then
    --...
  else
    --...
  end
end

for pl = PLAYER_1, PLAYER_8 do
  if GetPlayerState(pl) == PLAYER_ACTIVE then
    Trigger(PLAYER_ADD_HERO_TRIGGER, pl, 'UpdateHeroSpecs')
  end
end