[Agent Engine] Plugin not invoked when deploying with ADK 1.15.1

[RebeccaPangras]

I’m running into an issue where an ADK plugin works fine locally (via adk web and adk api_server), but does not get invoked when deployed to Agent Engine.

Environment

adk version: 1.15.1
google-cloud-aiplatform: 1.116.0
Using Vertex AI Agent Engine

agent.py

app = App(
    name="weather_app",
    root_agent=root_agent,
    plugins=[CustomPlugin()],
)

runner.py

from .agent import app
runner = Runner(
    app=app,
    session_service=session_service,
    artifact_service=artifact_service,
)

deploy.py

app = AdkApp(
    agent=root_agent,
    enable_tracing=True,
    plugins=[CustomPlugin()],
)

remote_app = agent_engines.create(
    agent_engine=app,
    display_name=root_agent.name,
    description=root_agent.description,
    requirements=[
        "google-cloud-aiplatform[adk,agent-engines]==1.116.0",
        "google-cloud-firestore==2.21.0",
        "google-adk==1.15.1",
    ],
    extra_packages=[
        "./weather_app",
    ],
)

custom_plugin.py

class CustomPlugin(BasePlugin):
    def __init__(self, name: str = "custom_plugin"):
        super().__init__(name)

    async def on_user_message_callback(self, *, context, user_message):
        logger.info("Plugin invoked")

How I tested

agent = agent_engines.get(RESOURCE_ID)
session_info = await agent.async_create_session(user_id="test-user-123")
events = agent.async_stream_query(
    user_id="test-user-123",
    session_id=session_info["id"],
    message="What is the weather today?",
)
async for event in events:
    print(event)

What I expect

The LoggerPlugin to be invoked in the deployed Agent Engine instance, same as when I run locally.

What actually happens
I verified by sending a http message to the deployed Agent Engine endpoint. The request succeeds, but
the plugin is not triggered at all in the deployed environment.

Works fine with adk web and adk api_server.

Related issue

#2636

Is plugin support in Agent Engine expected to work the same way as local ADK runners, or does it require different setup?