diff --git a/haystack/logging.py b/haystack/logging.py
index a5b8ef84d..fb0149a03 100644
--- a/haystack/logging.py
+++ b/haystack/logging.py
@@ -190,7 +190,10 @@ def patch_make_records_to_use_kwarg_string_interpolation(original_make_records:
     @functools.wraps(original_make_records)
     def _wrapper(name, level, fn, lno, msg, args, exc_info, func=None, extra=None, sinfo=None) -> Any:
         safe_extra = extra or {}
-        interpolated_msg = msg.format(**safe_extra)
+        try:
+            interpolated_msg = msg.format(**safe_extra)
+        except (KeyError, ValueError):
+            interpolated_msg = msg
         return original_make_records(name, level, fn, lno, interpolated_msg, (), exc_info, func, extra, sinfo)
 
     return _wrapper
diff --git a/releasenotes/notes/fix-logs-containing-json-1393a00b4904f996.yaml b/releasenotes/notes/fix-logs-containing-json-1393a00b4904f996.yaml
new file mode 100644
index 000000000..618dbc78e
--- /dev/null
+++ b/releasenotes/notes/fix-logs-containing-json-1393a00b4904f996.yaml
@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fixes logs containing JSON data getting lost due to string interpolation.
diff --git a/test/test_logging.py b/test/test_logging.py
index 2663b79e9..ce9607e04 100644
--- a/test/test_logging.py
+++ b/test/test_logging.py
@@ -490,6 +490,28 @@ class TestCompositeLogger:
             "module": "test.test_logging",
         }
 
+    def test_log_json_content(self, capfd: LogCaptureFixture) -> None:
+        haystack_logging.configure_logging(use_json=True)
+
+        logger = haystack_logging.getLogger(__name__)
+        logger.setLevel(logging.DEBUG)
+
+        logger.log(logging.DEBUG, 'Hello, structured: {"key": "value"}', key="logging", key1="value1", key2="value2")
+
+        output = capfd.readouterr().err
+        parsed_output = json.loads(output)
+
+        assert parsed_output == {
+            "event": 'Hello, structured: {"key": "value"}',
+            "key": "logging",
+            "key1": "value1",
+            "key2": "value2",
+            "level": "debug",
+            "timestamp": ANY,
+            "lineno": ANY,
+            "module": "test.test_logging",
+        }
+
     def test_log_with_string_cast(self, capfd: LogCaptureFixture) -> None:
         haystack_logging.configure_logging(use_json=True)