diff --git a/.github/workflows/pr-check-on-head-branch.yml b/.github/workflows/pr-check-on-head-branch.yml
index 62d762d72f..adb884d62c 100644
--- a/.github/workflows/pr-check-on-head-branch.yml
+++ b/.github/workflows/pr-check-on-head-branch.yml
@@ -78,8 +78,19 @@ jobs:
           if [[ ! "$BASE_SHA" =~ ^[0-9a-f]{40}$ || ! "$SINCE" =~ ^[0-9]+$ ]]; then
             stop "the master diff check reported no base commit, so nothing was merged or compiled"
           fi
-          git fetch --no-tags --no-write-fetch-head --shallow-since="$SINCE" origin "$HEAD_SHA" "$BASE_SHA" ||
+          # A shallow fetch starts a background maintenance run that rewrites .git/shallow, and the second shallow fetch
+          # below then fails with "shallow file has changed since we read it".
+          fetch() {
+            git -c maintenance.auto=false -c gc.auto=0 fetch --no-tags --no-write-fetch-head "$@" origin "$HEAD_SHA" "$BASE_SHA"
+          }
+          fetch --shallow-since="$SINCE" ||
             stop "the base commit $BASE_SHA could not be fetched, so nothing was merged or compiled"
+          # A time window records a merge commit itself as shallow when one of its parents predates the window, which
+          # drops the other, in-window parent too; a depth bound keeps both parents of every commit it covers.
+          if ! git merge-base "$HEAD_SHA" "$BASE_SHA" >/dev/null; then
+            fetch --depth=20 --filter=blob:none ||
+              stop "the base commit $BASE_SHA could not be fetched, so nothing was merged or compiled"
+          fi
           git -c user.name=pr-check -c user.email=pr-check@users.noreply.github.com merge --no-ff --no-edit -m "Merge $BASE_SHA into $HEAD_SHA" "$BASE_SHA" ||
             stop "the pull request cannot merge $BASE_SHA; merge master into it and push, then this check runs again"
           echo "merged=true" >> "$GITHUB_OUTPUT"