Before I put this into the bug reports, just asking here to make sure.

So from today's login reward premium bonus box, which is supposedly a legendary Avatar Item (Non-held), I got a pair of Zolnixi Ears which got a blue border, so... they're only considered rare right? Or does legendary simply mean that an item is recolourable? I'm a tad confused, so, just asking for clarification?

Hi! The avatar box rarities are Legendary -(Retired Avatar Item - Spring (Non-Held) )-, but the items inside this type of box aren't guaranteed to be.
For the Non-Held boxes, you're guaranteed to get a retired recolorable avatar item of some sort.
This is likely why the Boxes are considered Legendary quality without the items inside being guaranteed to be Legendary(The item can be Rare, Epic, or Legendary rarity so long as the item you receive is recolorable).
Any recolorable item from past Spring Fests can be obtained from these boxes, excluding Helds since those cannot be recolored.
So, you could get a recolorable Head, Outfit, Wings, Aspect, Cloak, or Tail item of any rarity so long as that particular item is recolorable.

With the Retired Avatar Item - Spring (Held) boxes, the items within those ones are guaranteed to be Legendary, but they're Helds only. So the item you get from the Held boxes are guaranteed to be Legendary(Yellow Border), unlike the Non-Held boxes.

Ah dang I see, thank you so much for the detailed explanation! I guess it makes sense but now I am frustratingly sitting on another dupe :p