diff --git a/.github/workflows/bootstrap.yml b/.github/workflows/bootstrap.yml
index 8f29357bd07..1b1a1922c9b 100644
--- a/.github/workflows/bootstrap.yml
+++ b/.github/workflows/bootstrap.yml
@@ -44,8 +44,13 @@ jobs:
         id: bootstrap-cache
         with:
           path: "/home/runner/work/cabal/cabal/_build"
-          key: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-${{ hashFiles(format('bootstrap/linux-{0}.json', matrix.ghc)) }}-${{ github.sha }}
-          restore-keys: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-${{ hashFiles(format('bootstrap/linux-{0}.json', matrix.ghc)) }}-
+          # Hash the .cabal files of the local packages into the key too:
+          # the bootstrap plans only reference local packages by name, so
+          # without this the caches of branches with different local package
+          # versions (e.g. master vs a release branch) collide and a stale
+          # _build/packages.conf gets restored. See #12311.
+          key: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-${{ hashFiles(format('bootstrap/linux-{0}.json', matrix.ghc), 'Cabal/Cabal.cabal', 'Cabal-syntax/Cabal-syntax.cabal', 'Cabal-hooks/Cabal-hooks.cabal', 'hooks-exe/hooks-exe.cabal', 'cabal-install-solver/cabal-install-solver.cabal', 'cabal-install/cabal-install.cabal') }}-${{ github.sha }}
+          restore-keys: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-${{ hashFiles(format('bootstrap/linux-{0}.json', matrix.ghc), 'Cabal/Cabal.cabal', 'Cabal-syntax/Cabal-syntax.cabal', 'Cabal-hooks/Cabal-hooks.cabal', 'hooks-exe/hooks-exe.cabal', 'cabal-install-solver/cabal-install-solver.cabal', 'cabal-install/cabal-install.cabal') }}-
 
       - uses: haskell-actions/setup@v2
         with:
diff --git a/bootstrap/bootstrap.py b/bootstrap/bootstrap.py
index 5638b13648c..30dcd9044d7 100755
--- a/bootstrap/bootstrap.py
+++ b/bootstrap/bootstrap.py
@@ -275,10 +275,12 @@ def hash_file(h, f: BinaryIO) -> SHA256Hash:
 PlanUnit = NewType('PlanUnit', dict)
 
 def bootstrap(info: BootstrapInfo, ghc: Compiler) -> None:
-    if not PKG_DB.exists():
-        print(f'Creating package database {PKG_DB}')
-        PKG_DB.parent.mkdir(parents=True, exist_ok=True)
-        subprocess_run([ghc.ghc_pkg_path, 'init', PKG_DB])
+    if PKG_DB.exists():
+        print(f'Deleting stale package database {PKG_DB}')
+        shutil.rmtree(PKG_DB)
+    print(f'Creating package database {PKG_DB}')
+    PKG_DB.parent.mkdir(parents=True, exist_ok=True)
+    subprocess_run([ghc.ghc_pkg_path, 'init', PKG_DB])
 
     for dep in info.builtin:
         check_builtin(dep, ghc)