diff --git a/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py b/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py
index 5f5073c916b68..644b0f5fa2a6d 100644
--- a/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py
+++ b/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py
@@ -892,15 +892,13 @@ def get_task_instance_count(
         query = query.where(TI.run_id.in_(run_ids))
 
     if task_group_id:
-        group_tasks = _get_group_tasks(dag_id, task_group_id, session, dag_bag, logical_dates, run_ids)
+        group_tasks = _get_group_tasks(
+            dag_id, task_group_id, session, dag_bag, logical_dates, run_ids, map_index
+        )
 
         # Get unique (task_id, map_index) pairs
-
         task_map_pairs = [(ti.task_id, ti.map_index) for ti in group_tasks]
 
-        if map_index is not None:
-            task_map_pairs = [(ti.task_id, ti.map_index) for ti in group_tasks if ti.map_index == map_index]
-
         if not task_map_pairs:
             # If no task group tasks found, default to checking the task group ID itself
             # This matches the behavior in _get_external_task_group_task_ids
@@ -1000,15 +998,18 @@ def get_task_instance_states(
     if run_ids:
         query = query.where(TI.run_id.in_(run_ids))
 
+    if map_index is not None:
+        query = query.where(TI.map_index == map_index)
+
     results = session.scalars(query).all()
 
     if task_group_id:
-        group_tasks = _get_group_tasks(dag_id, task_group_id, session, dag_bag, logical_dates, run_ids)
+        group_tasks = _get_group_tasks(
+            dag_id, task_group_id, session, dag_bag, logical_dates, run_ids, map_index
+        )
 
         results = results + group_tasks if task_ids else group_tasks
 
-    if map_index is not None:
-        results = [task for task in results if task.map_index == map_index]
     [
         run_id_task_state_map[task.run_id].update(
             {task.task_id: task.state}
@@ -1049,7 +1050,13 @@ def _is_eligible_to_retry(state: str, try_number: int, max_tries: int) -> bool:
 
 
 def _get_group_tasks(
-    dag_id: str, task_group_id: str, session: SessionDep, dag_bag: DagBagDep, logical_dates=None, run_ids=None
+    dag_id: str,
+    task_group_id: str,
+    session: SessionDep,
+    dag_bag: DagBagDep,
+    logical_dates=None,
+    run_ids=None,
+    map_index: int | None = None,
 ):
     # Get all tasks in the task group
     dag = get_latest_version_of_dag(dag_bag, dag_id, session, include_reason=True)
@@ -1070,6 +1077,7 @@ def _get_group_tasks(
             TI.task_id.in_(task.task_id for task in task_group.iter_tasks()),
             *([TI.logical_date.in_(logical_dates)] if logical_dates else []),
             *([TI.run_id.in_(run_ids)] if run_ids else []),
+            *([TI.map_index == map_index] if map_index is not None else []),
         )
     ).all()
 
diff --git a/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py b/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py
index 7f766ede71e4d..1ab690239fe6c 100644
--- a/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py
+++ b/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py
@@ -2336,6 +2336,7 @@ def add_one(x):
         ("map_index", "dynamic_task_args", "task_ids", "task_group_name", "expected_count"),
         (
             pytest.param(None, [1, 2, 3], None, None, 5, id="use-default-map-index-None"),
+            pytest.param(0, [1, 2, 3], None, None, 1, id="with-map-index-0-no-task-group"),
             pytest.param(-1, [1, 2, 3], ["task1"], None, 1, id="with-task-ids-and-map-index-(-1)"),
             pytest.param(None, [1, 2, 3], None, "group1", 4, id="with-task-group-id-and-map-index-None"),
             pytest.param(0, [1, 2, 3], None, "group1", 1, id="with-task-group-id-and-map-index-0"),
@@ -2853,6 +2854,15 @@ def add_one(x):
                 },
                 id="with-default-map-index-None",
             ),
+            pytest.param(
+                0,
+                [1, 2, 3],
+                None,
+                None,
+                {"-1": State.SUCCESS, "0": State.FAILED, "1": State.SUCCESS, "2": State.SUCCESS},
+                {"group1.add_one_0": "failed"},
+                id="with-map-index-0-no-task-group",
+            ),
             pytest.param(
                 -1,
                 [1, 2, 3],