diff --git a/.github/workflows/c++-code-formatting.yml b/.github/workflows/c++-code-formatting.yml
index 4d024b23..02ffeb8b 100644
--- a/.github/workflows/c++-code-formatting.yml
+++ b/.github/workflows/c++-code-formatting.yml
@@ -15,11 +15,13 @@ name: C++ code formatting reusable workflow
 permissions: {}
 
 env:
+  # GitHub also provides github.event.pull_request.{head,base}.sha, but these
+  # aren't always the latest commits on their respective branches (e.g. see
+  # https://github.com/AliceO2Group/AliceO2/pull/12499). Using them might lead
+  # to false positives in the errors we show.
   BASE_BRANCH: ${{ github.event.pull_request.base.ref }}
   PR_BRANCH: ${{ github.event.pull_request.head.ref }}
   PR_NUMBER: ${{ github.event.pull_request.number }}
-  HEAD_SHA: ${{ github.event.pull_request.head.sha }}
-  BASE_SHA: ${{ github.event.pull_request.base.sha }}
 
 jobs:
   clang-format:
@@ -61,14 +63,13 @@ jobs:
         id: clang_format
         run: |
           set -x
-          # $BASE_SHA is the latest commit on the branch the PR will be merged
-          # into, NOT the commit this PR derives from! For that, we need to find
-          # the latest common ancestor between the PR and the branch we are
-          # merging into.
-          base_commit=$(git merge-base HEAD "$BASE_SHA")
+          # $BASE_BRANCH is the branch the PR will be merged into, NOT the
+          # commit this PR derives from! For that, we need to find the latest
+          # common ancestor between the PR and the branch we are merging into.
+          base_commit=$(git merge-base HEAD "$BASE_BRANCH")
           # Find changed files, ignoring binary files.
           readarray -d '' commit_files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_SHA")
+            <(git diff -z --diff-filter d --name-only "$base_commit")
           [ ${#commit_files[@]} -gt 0 ] || { echo "No files to check"; exit 0; }
 
           # Check for invalid file extensions for C++ code.
@@ -156,6 +157,11 @@ jobs:
           # many commits back that point is.
           fetch-depth: 0
 
+      # Fetch the PR's head commit to find the common ancestor.
+      - name: Fetch PR branch
+        run: |
+          git fetch origin "$BASE_BRANCH" "pull/$PR_NUMBER/head:$PR_BRANCH"
+
       - name: Check copyright headers
         env:
           # The expected copyright notice. Comment markers ("//" or "#") are
@@ -177,7 +183,7 @@ jobs:
           # Find changed C++ and CMake files. Keep the file extensions in sync
           # with the ones in the "case" statement below!
           readarray -d '' files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_SHA" \
+            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_BRANCH" \
                        -- '*.cxx' '*.h' '*.C' '*.cmake' '*/CMakeLists.txt')
           # Run copyright notice check. Comment lines start with "//" for C++
           # files and "#" for CMake files.
@@ -314,7 +320,7 @@ jobs:
         run: |
           # Find changed files, ignoring binary files.
           readarray -d '' files < \
-            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_SHA" |
+            <(git diff -z --diff-filter d --name-only --merge-base "$BASE_BRANCH" |
                 while read -rd '' filename; do
                   file -bi "$filename" | grep -q charset=binary ||
                     printf "%s\\0" "$filename"