subtree merges lose prefix after rebase

Hi, folks!
why subtree merges lose prefix after an interactive rebase, is it a known
issue ? or I'am missing somethings:

$ for project in a b c ; do
    mkdir $project
    ( cd $project ; git init; touch ${project^} ; git add ${project^}
; git commit -m "add ${project^}" )
   done
$ tree
.
|-- a
|   `-- A
|-- b
|   `-- B
`-- c
    `-- C

$ mkdir tree && cd tree && git init
$ touch README && git add README && git commit -m "add readme"
$ for project in a b c ; do
  mkdir -p projects/$project
  git remote add -f $project ../$project
  git merge -s ours --no-commit $project/master
  git read-tree --prefix=projects/$project -u $project/master
  git commit -m "merge $project into tree"
  git pull -s subtree $project master
 done

$ tree
.
|-- projects
|   |-- a
|   |   `-- A
|   |-- b
|   |   `-- B
|   `-- c
|       `-- C
`-- README

$ git rebase -i -p a6d4e8e # this the hash of "merge b" commit
$ git commit --amend -m "merge b edit"
$ git rebase --continue
$ tree
.
|-- C
|-- projects
|   |-- a
|   |   `-- A
|   `-- b
|       `-- B
`-- README


Best regards
--
Zakaria ElQotbi
4096R/3DC2241A

"The secret of creativity is knowing how to hide your sources."
        - Albert Einstein