js file in custom module's library is not attached when big pipe is running
So it turns out that it was big pipe, but unrelated to my module. Elsewhere on the page a profile image is loaded to appear next to the user name, and if the user has none, a default image. It seems that if a 404 is returned (in this case because part of the oath did not yet exist to match the path supplied for the image), when big pipe goes to load it and gets the 404, downstream js is not loaded.