The Midwinter Blight progress bar at the Collection’s Table appears to be bugged…It shows constantly as full, and hovering over it to see how many you have/need to progress further shows no information at all. (Currently at Tier 3, and all the rewards were claimed)
tier 3 is as high as it goes i think.
You have completed all the tiers, there’s no more progress to be done :] tier 3 is the max