diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 80f9c76e9..50c6a5747 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -181,13 +181,13 @@ 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.field_info.annotation, BaseModel
-    ):
-        fields_to_extract = get_cached_model_fields(first_field.field_info.annotation)
-        return fields_to_extract
-    return fields
+    fields_to_extract = []
+    for f in fields:
+        if lenient_issubclass(f.field_info.annotation, BaseModel):
+            fields_to_extract.extend(get_cached_model_fields(f.field_info.annotation))
+        else:
+            fields_to_extract.append(f)
+    return fields_to_extract
 
 
 def get_flat_params(dependant: Dependant) -> list[ModelField]:
@@ -762,32 +762,25 @@ def request_params_to_args(
     if not fields:
         return values, errors
 
-    first_field = fields[0]
-    fields_to_extract = fields
-    single_not_embedded_field = False
     default_convert_underscores = True
-    if len(fields) == 1 and lenient_issubclass(
-        first_field.field_info.annotation, BaseModel
-    ):
-        fields_to_extract = get_cached_model_fields(first_field.field_info.annotation)
-        single_not_embedded_field = True
-        # If headers are in a Pydantic model, the way to disable convert_underscores
-        # would be with Header(convert_underscores=False) at the Pydantic model level
-        default_convert_underscores = getattr(
-            first_field.field_info, "convert_underscores", True
-        )
-
     params_to_process: dict[str, Any] = {}
 
+    fields_to_extract = [
+        (field, cached_field)
+        for field in fields
+        if lenient_issubclass(field.field_info.annotation, BaseModel)
+        for cached_field in get_cached_model_fields(field.field_info.annotation)
+    ]
+
     processed_keys = set()
 
-    for field in fields_to_extract:
+    for parent_field, field in fields_to_extract:
         alias = None
         if isinstance(received_params, Headers):
             # Handle fields extracted from a Pydantic Model for a header, each field
             # doesn't have a FieldInfo of type Header with the default convert_underscores=True
             convert_underscores = getattr(
-                field.field_info, "convert_underscores", default_convert_underscores
+                parent_field.field_info, "convert_underscores", default_convert_underscores
             )
             if convert_underscores:
                 alias = get_validation_alias(field)
@@ -809,27 +802,21 @@ def request_params_to_args(
             else:
                 params_to_process[key] = received_params.get(key)
 
-    if single_not_embedded_field:
-        field_info = first_field.field_info
-        assert isinstance(field_info, params.Param), (
-            "Params must be subclasses of Param"
-        )
-        loc: tuple[str, ...] = (field_info.in_.value,)
-        v_, errors_ = _validate_value_with_model_field(
-            field=first_field, value=params_to_process, values=values, loc=loc
-        )
-        return {first_field.name: v_}, errors_
-
     for field in fields:
-        value = _get_multidict_value(field, received_params)
-        field_info = field.field_info
-        assert isinstance(field_info, params.Param), (
-            "Params must be subclasses of Param"
-        )
-        loc = (field_info.in_.value, get_validation_alias(field))
-        v_, errors_ = _validate_value_with_model_field(
-            field=field, value=value, values=values, loc=loc
-        )
+        in_ = getattr(field.field_info, "in_", params.ParamTypes.query)
+
+        if lenient_issubclass(field.field_info.annotation, BaseModel):
+            loc: tuple[str, ...] = (in_.value,)
+            v_, errors_ = _validate_value_with_model_field(
+                field=field, value=params_to_process, values=values, loc=loc
+            )
+        else:
+            value = _get_multidict_value(field, received_params)
+            loc = (in_.value, field.alias)
+            v_, errors_ = _validate_value_with_model_field(
+                field=field, value=value, values=values, loc=loc
+            )
+
         if errors_:
             errors.extend(errors_)
         else: