diff --git a/.github/workflows/check-parent-branch.yml b/.github/workflows/check-parent-branch.yml
index 3a432e6bd..2ad310590 100644
--- a/.github/workflows/check-parent-branch.yml
+++ b/.github/workflows/check-parent-branch.yml
@@ -1,5 +1,5 @@
 # Block PRs targeting wip branches (v2.1-wip, v3.0-wip) when they are
-# behind their parent branch (v2.0 and v2.1-wip respectively).
+# behind their parent branches (v2.0, and v2.1-wip + v2.0 respectively).
 # This prevents unpropagated commits from accumulating, which leads to
 # costly backports of many commits generating large conflicts.
 # See https://github.com/TransmodelEcosystem/NeTEx/issues/982
@@ -21,13 +21,23 @@ jobs:
       - name: Verify base branch contains all parent commits
         run: |
           case "${{ github.base_ref }}" in
-            v2.1-wip) parent=v2.0 ;;
-            v3.0-wip) parent=v2.1-wip ;;
+            v2.1-wip) parents="v2.0" ;;
+            v3.0-wip) parents="v2.1-wip v2.0" ;;
           esac
 
-          behind=$(git rev-list --count "origin/${{ github.base_ref }}..origin/$parent")
-          if [ "$behind" -gt 0 ]; then
-            echo "::error::${{ github.base_ref }} is $behind commit(s) behind $parent. Merge $parent into ${{ github.base_ref }} first."
-            git log --oneline "origin/${{ github.base_ref }}..origin/$parent"
-            exit 1
-          fi
+          target="origin/${{ github.base_ref }}"
+          pr="${{ github.event.pull_request.head.sha }}"
+
+          # Count the commits of the target's parent branch (e.g. v2.0 for
+          # v2.1-wip) found neither in the target branch nor in this PR. An
+          # ordinary PR does not bring them in, so it stays blocked while the
+          # target lags behind. A backport PR does, so it is not blocked by
+          # the very lag it fixes.
+          for parent in $parents; do
+            behind=$(git rev-list --count "origin/$parent" --not "$target" "$pr")
+            if [ "$behind" -gt 0 ]; then
+              echo "::error::${{ github.base_ref }} is $behind commit(s) behind $parent. Merge $parent into ${{ github.base_ref }} first."
+              git log --oneline "origin/$parent" --not "$target" "$pr"
+              exit 1
+            fi
+          done