The implementation of OnGCE does an http.Get to "http://metadata.google.internal" using a client that has timeouts set to 750ms. Sometimes (I can repro semi-consistently when running inside a docker container) the func returns false, presumably due to timing out.

OnGCE also caches its result, so there's no way to retry. A workaround is to invent my own OnGCE func with longer timeouts, but I'd like to avoid doing that in the long term.