diff --git a/monai/inferers/merger.py b/monai/inferers/merger.py
index 31e9b5d632..f511f8b03e 100644
--- a/monai/inferers/merger.py
+++ b/monai/inferers/merger.py
@@ -286,32 +286,33 @@ def __init__(
         self.value_dtype = value_dtype
         self.count_dtype = count_dtype
         self.store = store
-        self.tmpdir: TemporaryDirectory | None
+        # Use separate tmpdir attributes so both TemporaryDirectory objects remain alive
+        # for the full lifetime of this instance. Previously a single `self.tmpdir` was
+        # overwritten when both value_store and count_store were None, causing the first
+        # TemporaryDirectory to be garbage-collected immediately.
+        self.value_tmpdir: TemporaryDirectory | None = None
+        self.count_tmpdir: TemporaryDirectory | None = None
 
         # Handle zarr v3 vs older versions
         is_zarr_v3 = version_geq(get_package_version("zarr"), "3.0.0")
 
         if is_zarr_v3:
             if value_store is None:
-                self.tmpdir = TemporaryDirectory()
-                self.value_store = zarr.storage.LocalStore(self.tmpdir.name)  # type: ignore
+                self.value_tmpdir = TemporaryDirectory()
+                self.value_store = zarr.storage.LocalStore(self.value_tmpdir.name)  # type: ignore
             else:
                 self.value_store = value_store  # type: ignore
             if count_store is None:
-                self.tmpdir = TemporaryDirectory()
-                self.count_store = zarr.storage.LocalStore(self.tmpdir.name)  # type: ignore
+                self.count_tmpdir = TemporaryDirectory()
+                self.count_store = zarr.storage.LocalStore(self.count_tmpdir.name)  # type: ignore
             else:
                 self.count_store = count_store  # type: ignore
         else:
-            self.tmpdir = None
             self.value_store = zarr.storage.TempStore() if value_store is None else value_store  # type: ignore
             self.count_store = zarr.storage.TempStore() if count_store is None else count_store  # type: ignore
 
         self.chunks = chunks
 
-        # Handle compressor/codecs based on zarr version
-        is_zarr_v3 = version_geq(get_package_version("zarr"), "3.0.0")
-
         # Initialize codecs/compressor attributes with proper types
         self.codecs: list | None = None
         self.value_codecs: list | None = None
@@ -322,50 +323,28 @@ def __init__(
             if codecs is not None:
                 self.codecs = codecs
             elif compressor is not None:
-                # Convert compressor to codec format
-                if isinstance(compressor, (list, tuple)):
-                    self.codecs = compressor
-                else:
-                    self.codecs = [compressor]
+                self.codecs = compressor if isinstance(compressor, (list, tuple)) else [compressor]
             else:
                 self.codecs = None
 
             if value_codecs is not None:
                 self.value_codecs = value_codecs
             elif value_compressor is not None:
-                if isinstance(value_compressor, (list, tuple)):
-                    self.value_codecs = value_compressor
-                else:
-                    self.value_codecs = [value_compressor]
+                self.value_codecs = value_compressor if isinstance(value_compressor, (list, tuple)) else [value_compressor]
             else:
                 self.value_codecs = None
 
             if count_codecs is not None:
                 self.count_codecs = count_codecs
             elif count_compressor is not None:
-                if isinstance(count_compressor, (list, tuple)):
-                    self.count_codecs = count_compressor
-                else:
-                    self.count_codecs = [count_compressor]
+                self.count_codecs = count_compressor if isinstance(count_compressor, (list, tuple)) else [count_compressor]
             else:
                 self.count_codecs = None
         else:
             # For zarr v2, use compressors
-            if codecs is not None:
-                # If codecs are specified in v2, use the first codec as compressor
-                self.codecs = codecs[0] if isinstance(codecs, (list, tuple)) else codecs
-            else:
-                self.codecs = compressor  # type: ignore[assignment]
-
-            if value_codecs is not None:
-                self.value_codecs = value_codecs[0] if isinstance(value_codecs, (list, tuple)) else value_codecs
-            else:
-                self.value_codecs = value_compressor  # type: ignore[assignment]
-
-            if count_codecs is not None:
-                self.count_codecs = count_codecs[0] if isinstance(count_codecs, (list, tuple)) else count_codecs
-            else:
-                self.count_codecs = count_compressor  # type: ignore[assignment]
+            self.codecs = codecs[0] if isinstance(codecs, (list, tuple)) else codecs if codecs is not None else compressor  # type: ignore[assignment]
+            self.value_codecs = value_codecs[0] if isinstance(value_codecs, (list, tuple)) else value_codecs if value_codecs is not None else value_compressor  # type: ignore[assignment]
+            self.count_codecs = count_codecs[0] if isinstance(count_codecs, (list, tuple)) else count_codecs if count_codecs is not None else count_compressor  # type: ignore[assignment]
 
         # Create zarr arrays with appropriate parameters based on version
         if is_zarr_v3: