Cannot change prefixes, item rarity bug.

I just tried to make a staff with one prefix, while keeping it rare, I used prefixes cannot be changed, and then scoured it, only to find out it made it magic.
I then find out this is the intended outcome.

Problem is I have an item I made a while back that did not obey this rule.

I strongly believe I used the exact same method as described above (I didn't want to risk losing the item so I know I didn't use an Orb of Annulment) but that time it didn't change the rarity.

However I did make this item a while back, so I may be mistaken.