Hopefully, the next big reveal will cover these transitions.
But it makes a bit of sense: more knowledge and thoughts written down in the first era, better start in the second. And if you really did it well, your science buildings will continue working, giving you - presumably - a great head start to the civs that have no free techs and no more science buildings.
I don't expect it to be perfect in any way, or really coherent once you start thinking about it. But if they can make it work and an interesting mechanic (eventually) that feels rewarding, fun, and makes the game more interesting after the first 100 or so turns, I'm fine with it that a free techs rubber band feels ... suboptimal...