To upgrade the Duskside Helm to Mythic level, currently four Serpentine Relics are required. Is this how it is supposed to be? Or is it supposed to be the current season’s Fallen Relic? Previous seasons would require the seasonal relic for the seasonal legendary item upgrades.
You’re correct this is a bug, it should use the Fallen Relic. I’ve reported it to the team. Thank you!
Update: We have a fix for this that should be going live before the weekend, or worst case scenario next week.
Thank you @Kafka for the quick fix!