When viewing the unowned weapons, the border color on ALL cards show as Common (white). If you click on ANY of the cards, it shows as Mythic. Most are actually Epic or Legendary. Note: If you filter by Rarity, it shows the correct order, but ALL the border colors are still white.
It does this for troops, too. Which is consistent, but still annoying. Troops at least have an almost-visible indicator to tell you their rarity.
Weapons do to. The background splash tells you the “Rarity”. I do agree its a super minor problem. Just saying you can figure it out from the image.