When you check the mission screen, the icon that shows the item that you will receive shows the items from the previous Seasonal Trials, as shown in the attachment. For example, the Seasonal Trial shown in the attachment has a Frost Slayer as a reward for completion, however the icon's picture shows a Sinister Sword (from Halloween Trials). This icon will still say the correct name of the item and you will still receive the correct item, only the visual icon is affected.

EDIT: It seems this only affects the Frost Slayer.