Seems to be a widespread issue for at least Xbox Series X - the Legendary Seal i got from the Lord of Hatred campaign (Vigorous Horadric Seal of Swiftness) is bugged. When you equip it, you can no longer equip more charms into empty slots. Furthermore, if the legendary seal is equipped and you unequip charms then you can no longer put any charms into open slots. The sound effects play as if you are slotting the charms - but nothing happens.
If you go back to a Rare Seal then I can slot charms again normally. If i switch to the legendary seal and open another charm slot - i cannot slot anything into the new opening.
Also, some seals don’t explicitly state how many slots they unlock - that should be corrected as well (including the Legendary Seal discussed in this post).
Please patch as soon as possible - thank you!
Update After Playing More:
It appears to be only the Seal that I named above - i got a new Legendary Seal and it works fine. So, not a huge deal, but was certainly scary when I thought no Legendary Seals would work.