Inconsistent Cache Response from Assets Served via Plug.Static

I’m having a really tricky time dealing with caching for my static assets. The responses are really inconsistent - I load up dev tools, reload the page and watch the responses. Sometimes, nothing is cached. Next reload, half are cached. Next reload, everything. Next reload, some. Next reload, nothing. I can see the etags on the items are the same every time.

What’s going on? I don’t want to keep serving the assets once they been delivered the first time.

Annoyingly, this doesn’t seem to happen in development - only in production.

EDIT: Nevermind. I can’t find any consistency.