Incorrect last modified fs value in Last-Modified header

I’m trying to debug a cache invalidation bug on a fastify app deployed to fly.

I’m serving files with fastify-static which uses the send under the hood. When things are working correctly, this sets the Last-Modified header to the last modified time of file based on the filesystem.

When running locally, the Last-Modified correctly reflects the last modified time of the files on disk, but when I deploy the app to fly with the heroku builder, the Last-Modified header always reports that the last modified time of the file as Tue, 01 Jan 1980 00:00:01 GMT.

Is there some kind of setting I need to turn on to get the correct last modified time during the build? Or does the container disabled last modified times on the file system or something? Any ideas appreciated.