To be sure that I understand this, the title (fTitle) of a TBranch of array type needs to be formatted as:
NAME_OF_BRANCH[NAME_OF_COUNTER]/[SINGLE_LETTER_TYPE]
and the title of its TLeaf must be
NAME_OF_BRANCH[NAME_OF_COUNTER]
(where NAME_OF_BRANCH is the name (fName) of the TBranch, NAME_OF_COUNTER is either the name of the counter or an integer for fixed-size arrays, and SINGLE_LETTER_TYPE is D for double, etc.).
If that’s right, Uproot is currently producing the wrong TBranch titles, but the correct TLeaf titles. I would need to add the [NAME_OF_COUNTER] part to the TBranch titles only. I just confirmed this pattern by looking at some files that ROOT created, but if I’m missing something, let me know!