Did Kylo Ren ever find out this bit of information?
That his mother, General Leia, survived the earlier attack.
Lots of spoilers
The movie evokes Return of the Jedi (when Luke and Vader have a moment through The Force while on passing ships) in the moments before the attack. Kylo Ren can sense his mother on the ship (the camera switched back and forth between the two just as it did in ROTJ). He didn't hesitate with his father, but he can feel his mother through The Force... and then another fighter does the deed for him.
Even with that distance between them after the attack, it's a pretty safe bet he could feel her when she uses The Force to reach out and pull herself back on board. In the off chance Ren didn't notice, there's almost no chance Snoke didn't, and he would have used it to make another dig at Kylo Ren (and gin up that anger Sith thrive on, just as he did in saying he was mistaken about Ren being another Vader).
The film's junior novelisation confirms that after his famous duel with Luke, Kylo Ren senses both Rey and Leia boarding the Falcon.
He picked up two small objects from the ground. A pair of chance cubes, strung together. Ren knew the dice well. They had been Han’s, hanging in the cockpit of The Millennium Falcon.
Ren glimpsed the ship through the Force. Rey and Leia were boarding. Both seemed upset by something.
We do not ever see him learn of this information on-screen.
That said, we have seen that people who are strong in the Force tend to know when someone close to them dies.
Kylo Ren obviously still has a connection to Leia, but we have no way of knowing if that connection was maintained after
Leia was ejected into space.
It's certainly possible that Kylo knew
She had survived that
but it isn't clear to me if he was aware that she
Made it to the salt-covered planet with the transports.
Leia certainly doesn't seem to think he will be aware, given that she
remains confident that the First Order will not realize the Rebels evacuated to the secret base
but DOES, in her later conversation say
that she knows Kylo is beyond salvation, and is entirely committed to the First Order.
Thus, we must assume that she had reason to think he expected her to be dead or was otherwise unable to locate her via the Force. Perhaps she took steps to hide her presence?
This is not to say, however, that Kylo would not become aware of
after the events of the movie. Given that the Rebel leadership tends to be known by the Empire/First Order leadership, it's fair to assume that the renewed Rebellion would make no secret of her status.