Since there doesn't seem to be a scholarly consensus on the issue, I'll give my own opinion, which is based on things like εἶδος δὲ λέγω τὸ τί ἦν εἶναι 1035b, so it looks like Aristotle means the "form" by τὸ τί ἦν εἶναι. So I think imperfect is simply due to the formal cause being thought of in terms of being anterior in time. I'm thinking of things like the example with the acorn and oak tree, so the "essence" of the acorn/oak was that which was there to make it be what it is. So basically I think it's just a normal past tense.
I was looking for similar usages and found a reference to a passage De Partibus Animalium that says φανερὸν ὅτι τὸ αἷμα ὡδὶ μὲν ἔστι θερμόν, οἷόν τι ἦν αὐτῷ τὸ αἵματι εἶναι, where again I just think it's natural to think of the formal cause as being prior in time.
But I'm analyzing the phrase so that εἶναι goes with ἦν and I'm not sure on that point. The one suggestion I saw is that τὸ τί ἦν εἶναι corresponds to things τὸ ἀγαθῷ εἶναι with τί ἦν representing the dative in this construction, but then I don't know what the imperfect would mean. I find lots of references to the "philosophical imperfect"
but even if this is a legitimate category, I don't really see how it applies to Aristotle's phrase.