Not criticising the comic, I love me some Hachiko, but I am curious: has anyone actually forgotten a childhood friend? Is this something that happens, ever? I mean, I've forgotten random classmates, but I'm pretty sure I remember all of my close friends from when I was 5, and I didn't even promise to marry them. This trope is strangely common for how unbelievable it is to me, but maybe other people have different experiences?
I personally never been in such a situation, I think I remember most of the the children I was close with since elementary school, but I never happened to meet one I haven't seen in 10 years or so, so I might be wrong.
On the other hand, I know a guy who remembers very well the events and people he knew as a child, but is absolutely terrible at recognizing faces, so even when in his last year of high school he was accosted by a girl who was with us in middle school, he was like "I'm sure I know you from somewhere, but where was it..." until I reminded him "She was class rep in 8th grade." and he remembered "Ah, Laura. You were best friends with..." etc...
Thankfully she didn't take it badly.
Though I think the essence of this trope is that, as demonstrated by different commenters, people starts having clear recollection at different points of their life, but don't realize this is the case unless they compare. So childhood friend A will assume that everyone her age remember things from kindergarten since she does when actually childhood friend B has only vague memories before elementary school.