So it turns out that when you build redis on my system, it sets --with-jemalloc-prefix=je_, which means that all of jemalloc's public APIs become prefixed with the string je_ (or JE_)

Running export JE_MALLOC_CONF=narenas:40 then results in the expected behavior.

The prefix behavior is described here : https://github.com/jemalloc/jemalloc/blob/dev/INSTALL.md