(Rakan main here noticing this the instant I played him) In a recent game when I was playing Rakan I noticed a bug with his shielding ability: Usually when you cast his shield the first time it costs mana, and you are able to recast it for free. When I was low on mana but had just enough to cast my shield, I was unable to cast it a the second time on an ally.
Let's say that Rakan has 70 mana left (which is just enough to cast Battle Dance before you level up that ability) and he shields an ally, using up the rest of his mana, he should still be able to cast his shield again to an ally (whether if it's the same person or a different ally) for free. What I found was that when Rakan uses the last of his mana to shield a target ally, I can't use his shielding ability again even though I'm supposed to be able to recast it for free, as if he needs mana to shield someone the second time.
I even tried it out in the practice tool, and for Rakan to be able to cast his shield twice (the second time for free) he needs to have at least 140 mana. If his mana bar has less than 140 mana then he is only able to cast his shield once.