Platform, device version and operating system:
PC
What you were expecting to happen, and what actually happened:
Adventure tasks share a difficulty progression based on their rarity:
- Common: 500, 1000, 1500
- Rare: 1000, 1500, 2800
- Ultra-Rare: 1000, 1500, 5200
- Epic: 1500, 5200, 9600
- Legendary: 1500, 5200, 7700, 11000
- Mythic: 1500, 5200, 7700, 11000
The last two (seen on Search for Keys V and Search for Keys VI) seem to be a cut & paste error, mythic tasks are likely intended to be harder than legendary task (or, looking at it the other way around, legendary tasks easier than mythic tasks).