The episode was "Lethe" (DSC). She was angry at him when she found out, but then they began working toward an understanding from there.Okay, must have ignored/forgotten that - but that doesn't actually explain why Spock doesn't talk to either of them or why Burnham isn't angry at Sarek.
Obviously, it's yet to be clarified (and most likely will be from here), but Spock's estrangement from them could have something to do with the fact that Sarek melded with Burnham to save her life when she was a child, leaving part of his katra with her, an intimacy that Spock would never get to experience with his father in life, per "Unification II" (TNG). Spock was always deeply insecure about his father's acceptance of him, as we can see not only from that episode, but from his "secret pain" as depicted in the fifth film as well. Ironically, despite Sarek choosing Spock over Michael, to young Spock it may rather have seemed that he chose her over him. There might be more to it, of course, but we'll see if that's at all in the ballpark...
-MMoM
