after a merge down between a parent and child stream,if I rollback a specific folder, I cant see the mergedown option anymore between the two streams

Question

In my local stream workplace, I first did a merge down from a parent stream and then used rollback to revert the merge down on a specific folder.

The rollback worked on that folder, however since the child dev stream and the parent stream are different again, I dont see a merge down option again (say I want to merge down the two streams again).

Why cant I see the merge down option again?