In multi-thread Snapshots, we sometimes get warnings printed for each TBranchElement that is active in the input chain but is not present in the output tree.
The warnings are printed by TTree.cxx:3253, in TTree::CopyAddresses.
CopyAddresses loops over all branches of the main tree, and prints a warning if
an *active* branch is not present in a clone.
I cannot think of a general dataframe-side solution, but in the case in which the input tree has no friends we could avoid adding the output trees as clones of the input tree altogether, sidestepping the issue.
To be understood why this does not happen in a single-thread Snapshot.