Custom banners and shields made with uncommon and rare banner patterns have incorrect rarity

Banners and shields made with uncommon and rare banner patterns still have common rarity.

Steps to reproduce:

1. Obtain a banner, a dye and a Thing Banner Pattern

2. Use them in a loom to get a banner with the Thing pattern applied. Note the banner's rarity

3. Obtain a shield

4. Apply the banner to the shield. Note the shield's rarity

Observed result:
Both the banner and the shield with the banner applied have the Common rarity.

Expected result:
Both the banner and the shield with the banner applied have the Rare rarity.