Trying to dye banners using a loom inventory that was created with Server.createInventory(InventoryHolder, InventoryType, String) does not allow you to successfully modify the banner with dyes.

To test, upload the BugTestingPlugin that is attached and run /openinventory LOOM.

The command will open the loom inventory and give you a banner with 64 red dye. Place the banner and dye in the loom inventory, and select any pattern and click the result.

One of two results is expected, either:

1) If left clicked, the pattern will reset and nothing will be added to the banner.

or

2) If shift clicked the banner will go into your inventory. Once left clicked again, it will disappear.

To test the intended result, you can go into the creative inventory and get a loom, a banner, and dye. Place the loom, right click on it, and follow the steps above for adding a pattern. The banner should then have the pattern added with no issue.