Hello. From SSG's page on
"Dusts of Mythic and Reaper Power provide a Mythic bonus to specific items as indicated in their name. The dust item applies a benefit through 4/6/27 to their relevant item type. The benefit does not expire once applied. The Dusts come in Glove, Boot, and Cloak varieties."
Do you see the omission? These Dusts of Mythic and Reaper Power do not "provide a Mythic bonus"; they respectively provide a Mythic or
Reaper bonus depending on which dust is used.