First of all, JKR said that Lily's case was very unique. She actually said something like this never happened before (can't find the quote, I'll edit later). Meaning the circumstances have to be very specific for this protection effect to occur. After all, Lily and Leta are not the only brave people who died for their loved when they had a chance to flee and save themselves, are they? Similar case with James.
JKR: Don't you want to ask me why James's death didn't protect Lily and Harry? There's your answer - you've just answered your own question - because she could have lived - and chose to die. James was going to be killed anyway. Do you see what I mean? I'm not saying James wasn't ready to; he died trying to protect his family, but he was going to be murdered anyway. He had no - he wasn't given a choice, so he rushed into it in a kind of animal way. I think there are distinctions in courage. James was immensely brave. But the caliber of Lily's bravery was, I think in this instance, higher because she could have saved herself. Now any mother, any normal mother would have done what Lily did. So in that sense, her courage too was of an animal quality but she was given time to choose. James wasn't. It's like an intruder entering your house, isn't it? You would instinctively rush them. But if in cold blood you were told, "Get out of the way," you know, what would you do? I mean, I don't think any mother would stand aside from their child. But does that answer it? She did very consciously lay down her life. She had a clear choice.
ES: And James didn't.
JKR: Did he clearly die to try and protect Harry specifically given a clear choice? No. It's a subtle distinction and there's slightly more to it than that but that's most of the answer.
Hence we see only two cases of this sacrifice magic to come into action: Lily's sacrifice to protect Harry and then Harry's sacrifice to protect his friends.
Now I see how someone can assume that Leta also had a choice - a choice either to flee or to join Grindelwald. But it was not that kind of choice. Arguably anyone, including James, have such choices - try to flee, try to survive by fighting etc. The specific nature of Lily's and Harry's sacrifices was that they didn't try to change what was going to happen - they died willingly, without fighting. While they had clear choice not to.
I don't see Leta was doing the same. She fought, she tried to influence the situation and she didn't have much choice or possibilities to survive as they all were on the edge of being killed anyway.