I do think she "had a reason" and personally I think its plot. Waking Mia up before getting forced into hero (mc can't leave) makes the plot alot "easier".As longas Ella is motivated by her brand of self-righteousness, she'll do whatever she does. Its not hard to think that Ella is playing 8 moves ahead
In universe though what is the "8 moves" ahead for healing Mia a couple of weeks early? MC has the ability to not thank her so its not like she's doing it to get in MC good graces which relies on you wanting/caring about Mia in the first place.
Obviously she could be playing 40d chess and knows Mia's entire future and that she'll be absolutely required for her plan but there's just as much chance she isn't (50/50)