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It will get adapted sooner or later. Nowadays changes to the order are less common, but some cases are for some reason adapted out of order and way later. For example, the Shinichi Heiji flashback was adapted over two years after the manga release and the Agasa's First Love cases was adapted over three years after the manga release. So really, it's not something to be concerned about. Everything in the manga is going into the anime eventually (and the pre-school flashback has gone into the anime considering the Ending Futari no Byoshin). Fans and some Japanese media magazines are speculating that they will adapt the pre-school flashback in March, because that's the time when cherry trees blossom in Tokyo. And even if they are not, you should relax a bit. It will come eventually.