Can't wait for the next patch, which supposedly may fix this.
I think the cause is, since the AI's been improved, they research tech faster, (Since their economy is better and they don't death-spiral anymore) which also means they discover L-gate insights faster, which means they open the L-gates earlier than before. (Before, I've never seen them open the L-gates before endgame crisis hits on default settings and difficulty)
I think maybe it should depend on the AI personality of the empire how much they want to open the L-gates. Isolationist ones likely wouldn't really care for the L-gates, while a xenophile materialist should want to open them to see what's on the other side out of curiosity.
I also think maybe there should be some galactic community resolutions for the L-gates. Not sure what (Choosing to research an L-gate insight puts you in breach of Galactic Law for a bit?) but I feel you should be able to use the galactic community to try and discourage empires from opening it. I can't think of a good in-universe justification, but from a meta perspective as players, we obviously know if they're opened it can burn down the entire galaxy. (Or maybe you don't want people from obtaining nanites for whatever reason)