Session files being stored in /tmp instead of $PREFIX/var/session_data

I’m upgrading RT from 3.6.6 to 3.8.1. I’ve gotten most things working
just fine, except for the fact that for some reason the Apache-Session
files are being written to /tmp instead of ./var/session_data.

I double checked permissions, and they are correct.
In system configuration RT::MasonSessionDir is set to
/usr/local/rt/var/session_data (as it should be)

Am I missing something simple?