For me it was, a noted before, shown not as a sticker on the box but printed on it.
View attachment 28749
IIRC It also noted that on the advert on Amazon's advert.
I also know that when I bought my AX88U that too shows the same, again the advert at Amazon also states that.
Asus were happy to help me a mine was just within the 3 years warranty.
Perhaps the warranty length can be a regional thing, as I'm in the UK..?
Maybe someone else might comment, are you in the US..?
Yes,I'm in the US. I'm pretty sure that my box looked exactly like yours. I threw away the box a while back,not thinking I would need it. After Asus responds,I may show them a Picture of yours to show them that they do have the 3 yr warranty on it and to explain it.