It Probably couldn't... I mean if it can see the future, it could Probably see the time twins coming back, and them altering time, so it probably avoids that... Except the Time Twins never altered anything Master Wu did, and all he did was make the designs different, so the future reflection would still be the same only as the Updated Nya and Jay
Unless the cave only showed the possible future; that does happen in scenarios like these. Don’t get me wrong, I’m hoping for Nya’s return as well, but I don’t think the future reflections necessarily guarantee anything.