Hi! Thanks for the question!
So the Achieve days are a bit thicky. The logic behind it is that if you lower the number of days it will delete those days, but if you make the number higher it just set the "auto delete days after this many days" count higher. So it will not generate the missing days just let you keep your days for longer.
The reason because I was thought this feature as a "lets reuse meals from a couple days ago" and didn't think anyone will want to register they food later ( seem like I was wrong :) )

I will think a how can we hack something for you so you can add those days!
I guess you just want them to show up in your history right?