Essence encounter dropped the original reward after modifying it with a vaal orb

In the Glacial Tarn, map of the 3rd interlude, there was an essence encounter with a checkpoint beside it. It had an "Essence of Ice" as reward, and it offered me to use a vaal orb that I picked minutes before.
When I used it through clicking the icon the reward changed to a "Greater Essence of Abrasion".
After killing the yellow mob, it dropped the "Essence of Ice" instead of the new one.
I don't know if happend because it is scripted for being a "checkpointed" encounter in an ice map, I haven't tested it with other encounters yet.