diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 84dfa4d03..83a7a44c5 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -212,11 +212,14 @@ def get_flat_dependant(
 def _get_flat_fields_from_params(fields: List[ModelField]) -> List[ModelField]:
     if not fields:
         return fields
-    first_field = fields[0]
-    if len(fields) == 1 and lenient_issubclass(first_field.type_, BaseModel):
-        fields_to_extract = get_cached_model_fields(first_field.type_)
-        return fields_to_extract
-    return fields
+    result = []
+    for field in fields:
+        if lenient_issubclass(field.type_, BaseModel):
+            fields_to_extract = get_cached_model_fields(field.type_)
+            result.extend(fields_to_extract)
+        else:
+            result.append(field)
+    return result
 
 
 def get_flat_params(dependant: Dependant) -> List[ModelField]: