Currently, when I enters a Math block, my focus is on the parent level by default. I habitually arrow left and right until I understand that I will need to go a level deeper. I would argue that a user's principal goal in going into a Math block is to explore it granularly which can only be achieved starting at the level below the parent level. Therefore, that should be the default level the user lands on when they Enter into Math content.
Note: I can see the case to default to the bottommost level too - so all the way down and deep into the Math - but I won't advocate for it myself.