We know that a living being is just not only DNA, but also its environment; now on top of that add all the big complexity of human social behaviour and gender into it. I doubt they will find a "cause" to trans-ness.
As a thought experiment, it seems hard to intentionally design a drug that changes what year puberty onset starts at without having some impact on LGBTQ+ prevalence.
I can tell you my attraction has been consistent before and after puberty. And trans friends speak of having gender dystopia at a very young age.
More openly trans people for sure. But who knows how many kept it bottled up and hidden, perhaps even from themselves, throughout history?