Well. That's his fault, isn't it, but then again in the moment Cassian isn't sure that Jyn would do well with hearing after your sacrifice on Scarif as a primer to her life and the ripple effect it's had on how history continued afterward. He can't bring himself to regret it too much.

"He can talk about it later, when things aren't so...up in the air. With what Poe remembers, with what you remember." Because Cassian is well-aware who told the droid not to speak on the subject, thanks.