diff --git a/dpdata/system.py b/dpdata/system.py
index 18b16aaf..36a01111 100644
--- a/dpdata/system.py
+++ b/dpdata/system.py
@@ -480,7 +480,10 @@ def append(self, system: System) -> bool:
             return False
         elif not len(self.data["atom_numbs"]):
             # this system is non-converged but the system to append is converged
-            self.data = system.data.copy()
+            # A shallow dict copy would still alias all arrays and lists in
+            # ``system.data``.  The first append must have the same ownership
+            # semantics as subsequent append operations.
+            self.data = deepcopy(system.data)
             return False
         if system.uniq_formula != self.uniq_formula:
             raise RuntimeError(
@@ -1492,8 +1495,10 @@ def from_dir(
         type_map: list[str] | None = None,
     ):
         multi_systems = cls()
+        # Do not prepend ``./``: doing so turns an absolute directory into a
+        # relative pattern and silently yields no matches.
         target_file_list = sorted(
-            glob.glob(f"./{dir_name}/**/{file_name}", recursive=True)
+            glob.glob(os.path.join(dir_name, "**", file_name), recursive=True)
         )
         for target_file in target_file_list:
             multi_systems.append(
diff --git a/tests/test_multisystems.py b/tests/test_multisystems.py
index fd9b278d..7c363c2a 100644
--- a/tests/test_multisystems.py
+++ b/tests/test_multisystems.py
@@ -51,6 +51,19 @@ def test_str(self):
         )
 
 
+class TestMultiSystemsFromDir(unittest.TestCase):
+    def test_absolute_directory_matches_relative_directory(self):
+        relative = dpdata.MultiSystems.from_dir(
+            "poscars", "OUTCAR.h2o.md", fmt="vasp/outcar"
+        )
+        absolute = dpdata.MultiSystems.from_dir(
+            os.path.abspath("poscars"), "OUTCAR.h2o.md", fmt="vasp/outcar"
+        )
+        self.assertEqual(len(relative), 1)
+        self.assertEqual(len(absolute), len(relative))
+        self.assertEqual(absolute.get_nframes(), relative.get_nframes())
+
+
 class TestMultiSystemsAdd(unittest.TestCase, CompLabeledSys, MultiSystems, IsNoPBC):
     def setUp(self):
         self.places = 6