I think the reason he doesn't remember might have to do with her immortality. Maybe she gave him a bit of immortality juice before he died via kiss, but he doesn't remember.
@b4nji
There was a chapter not long ago where they spread Tokiko's ashes.
Although given her skateboard comment it sounds like maybe she died doing something stupid rather than "of old age"?
Remember chapter 140.5? Yeah it says her grandma died July 20th, we see here that on the calendar it says July 7th. For some reason 140.5 came out before this which defuses the drama a lot but kinda ruins the grandma's death.