When there are multiple sources of the same aura nearby, the game does not automatically prioritize the highest level of them.

For example, if an allied Beastmaster with level 1 aura picks up an illusion rune, then sends the illusion to an ally, and then levels up his aura to 2, and then moves to the ally as well. The ally will still be affected by the level 1 aura, since the illusion was there first, so it placed the level 1 aura buff. The stronger level 2 aura of Beastmaster is ignored. Even worse, even after the illusion died and is long gone, the aura buff will stay on level 1. Beastmaster's level 2 aura keeps the level 1 buff up on the ally. In order for the ally to be affected by the level 2 aura, it first has to fully lose the buff, and then re-gain it. The ally has to leave the aura radius and re-enter it for it to update.

This is even worse for heroes with global auras. Here is a scenario that can cause CM's aura to not update for some allies until they actually die.

1. Allied CM has level 1 Arcane Aura, you are affected by it
2. Allied CM picks up an illusion rune
3. Allied CM dies, the aura buff is now kept up by the illusions
4. While dead, CM levels up aura to level 2
5. Before the illusions expire, CM respawns
6. The illusions expire and disappear
7. You still are affected by the level 1 aura, even though CM has it on level 2 now
8. The aura buff will stay level 1 on you until you somehow lose it (by dying, most commonly). Even when CM levels it up to 3 or 4, it stays on level 1 on you until losing and regaining the buff.

So for heroes with local auras, it's not that big of an issue (although still annoying, having to walk 900 range away from your aura ally in order for it to apply current level), but for heroes with global auras, it can keep their aura on low level for a much longer time and you have no good way of updating it (dying, or turning invulnerable, something a lot of heroes can't).