Damn, we're really seeing more evidence towards this past Allen had. I mean we already knew he was de-aged at some point, Neah confirmed this with his own confusion at Allen's apparent age being too young, and here we have Allen missing memories and actual trying to remember things. I guess once he "became" Allen and travelled with Mana, he probably pushed these thoughts down because he was so happy, and afterwards he changed personality again to be more like Mana, likely further pushing this down. It might not be for some time still due to the rate these chapters come out (I can't complain though considering it's better getting something every 3 months rather than not knowing if it ever would continue) but we might one day see what this "original" Allen was like when he's forced to confront these old memories.