When does PluginContainer.transform get called in Vite 6?

[grahse]

I'm trying to upgrade to Vite 6 and have a custom plugin where, within a configureServer hook, I'm wrapping the PluginContainer.transform hook. For example:

{
   name: "custom-plugin",
   apply: "serve",
   enforce: "post",
   configureServer(server) {
      const wrappedTransform = server.pluginContainer.transform;
      server.pluginContainer.transform = function () {
         // This never gets called.
      }
   }
}

The issue is that the transform hook never seems to get called. This works with Vite 5, so I'm wondering if something changed in Vite 6 with how/when it gets called?