Can't load extension because of mimetype mismatch

I’m trying to load an extension that I’ve developed locally, but when I launch JupyterLab and open a notebook I’m getting this error:

The resource from “http://localhost:8888/lab/extensions/jupyterlab-ihaskell/static/remoteEntry-9c60627a05d7fcc32ccf.js” was blocked due to MIME type (“text/html”) mismatch (X-Content-Type-Options: nosniff).

The extension seems to correctly show up in the extension manager and this workflow was fine in previous versions. Is there something I’m missing?

My current theory is that my extension is not being loaded correctly, but I’m setting JUPYTER_PATH which worked like I expected in previous versions. Has something changed here with JupyterLab 4?