I'm using a React app with module federation and webpack. I've recently updated all the micro UIs and the host from react 18 to react 19.

The production build works fine with no warnings or errors, but when running in development I get the following error:

The current testing environment is not configured to support act(...)

I know I've seen this in testing but this is when running the app using webpack serve --open --mode development, and I never saw this when using React 18.

I've checked the dependencies on both the host and the micro UIs. Non of the reference any testing libs. I've tried setting the IS_REACT_ACT_ENVIRONMENT variable to both true and false in the way this question does, but that issue comes about when running tests.

I suspect the error is happening when webpack is loading other modules, but I don't have a lot of information to confirm that.