diff --git a/src/spatialdata/_core/query/relational_query.py b/src/spatialdata/_core/query/relational_query.py
index 0580af73..7ef7c1a0 100644
--- a/src/spatialdata/_core/query/relational_query.py
+++ b/src/spatialdata/_core/query/relational_query.py
@@ -242,6 +242,10 @@ def _get_masked_element(
     return element.loc[mask_values, :]
 
 
+def _region_as_str_if_list_of_len_one(region: list[str]) -> str | list[str]:
+    return region if len(region) > 1 else region[0]
+
+
 def _right_exclusive_join_spatialelement_table(
     element_dict: dict[str, dict[str, Any]],
     table: AnnData,
@@ -279,9 +283,10 @@ def _right_exclusive_join_spatialelement_table(
     exclusive_table = table[keep, :] if has_match and keep.any() else None
     _inplace_fix_subset_categorical_obs(subset_adata=exclusive_table, original_adata=table)
     if exclusive_table is not None:
-        exclusive_table.uns[TableModel.ATTRS_KEY][TableModel.REGION_KEY] = (
+        exclusive_table.uns[TableModel.ATTRS_KEY][TableModel.REGION_KEY] = _region_as_str_if_list_of_len_one(
             exclusive_table.obs[region_column_name].unique().tolist()
         )
+        TableModel.validate(exclusive_table)
     return element_dict, exclusive_table
 
 
@@ -383,9 +388,10 @@ def _inner_join_spatialelement_table(
 
     _inplace_fix_subset_categorical_obs(subset_adata=joined_table, original_adata=table)
     if joined_table is not None:
-        joined_table.uns[TableModel.ATTRS_KEY][TableModel.REGION_KEY] = (
+        joined_table.uns[TableModel.ATTRS_KEY][TableModel.REGION_KEY] = _region_as_str_if_list_of_len_one(
             joined_table.obs[region_column_name].unique().tolist()
         )
+        TableModel.validate(joined_table)
     return element_dict, joined_table
 
 
@@ -466,9 +472,10 @@ def _left_join_spatialelement_table(
     joined_table = table[joined_indices.tolist(), :].copy() if joined_indices is not None else None
     _inplace_fix_subset_categorical_obs(subset_adata=joined_table, original_adata=table)
     if joined_table is not None:
-        joined_table.uns[TableModel.ATTRS_KEY][TableModel.REGION_KEY] = (
+        joined_table.uns[TableModel.ATTRS_KEY][TableModel.REGION_KEY] = _region_as_str_if_list_of_len_one(
             joined_table.obs[region_column_name].unique().tolist()
         )
+        TableModel.validate(joined_table)
     return element_dict, joined_table
 
 
diff --git a/tests/core/query/test_relational_query.py b/tests/core/query/test_relational_query.py
index 737c9c51..4f87098a 100644
--- a/tests/core/query/test_relational_query.py
+++ b/tests/core/query/test_relational_query.py
@@ -256,13 +256,13 @@ def test_join_updates_spatialdata_attrs(sdata_query_aggregation):
     _, table = join_spatialelement_table(
         sdata=sdata, spatial_element_names="values_circles", table_name="table", how="left"
     )
-    assert table.uns["spatialdata_attrs"]["region"] == ["values_circles"]
+    assert table.uns["spatialdata_attrs"]["region"] == "values_circles"
 
     # inner join on a single element
     _, table = join_spatialelement_table(
         sdata=sdata, spatial_element_names="values_circles", table_name="table", how="inner"
     )
-    assert table.uns["spatialdata_attrs"]["region"] == ["values_circles"]
+    assert table.uns["spatialdata_attrs"]["region"] == "values_circles"
 
     # right_exclusive join: pass a truncated circles element so some table rows have no match.
     # values_circles has 9 instances (0-8); keep only 5 → 4 table rows are exclusive.
@@ -275,7 +275,7 @@ def test_join_updates_spatialdata_attrs(sdata_query_aggregation):
     )
     assert table is not None
     assert table.n_obs == 4
-    assert table.uns["spatialdata_attrs"]["region"] == ["values_circles"]
+    assert table.uns["spatialdata_attrs"]["region"] == "values_circles"
 
     # original table metadata must be unchanged
     assert set(sdata["table"].uns["spatialdata_attrs"]["region"]) == {"values_circles", "values_polygons"}
@@ -1008,6 +1008,7 @@ def test_labels_table_joins(full_sdata):
 def test_points_table_joins(full_sdata):
     full_sdata["table"].uns["spatialdata_attrs"]["region"] = "points_0"
     full_sdata["table"].obs["region"] = ["points_0"] * 100
+    full_sdata["table"].obs["region"] = full_sdata["table"].obs["region"].astype("category")
 
     element_dict, table = join_spatialelement_table(
         sdata=full_sdata,