MySQL tmpdir refuses to change from default - no errors

I'm currently migrating a local LAMP project to an ubuntu AWS instance, but a small and frustrating issue is holding up the process. MySQL is not changing it's tmpdir, despite it being defined in my.cnf. I've done this before on my local ubuntu machine, but something is preventing the tmpdir from being changed on this ubuntu instance.

grep "tmpdir" /etc/my.cnf

yields

tmpdir = /data/tmp/

Where it should be.

When I restart mysql however,

SHOW VARIABLES LIKE 'tmpdir';

yields

+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| tmpdir        | /tmp  |
+---------------+-------+
1 row in set (0.00 sec)

Stranger still, I also have a custom-defined data directory that loads completely fine:

SHOW VARIABLES LIKE 'datadir';

+---------------+------------------+
| Variable_name | Value            |
+---------------+------------------+
| datadir       | /data/mysql/ |
+---------------+------------------+
1 row in set (0.00 sec)

I've tried playing with permissions - I tried setting /data/tmp to 755, 700, and 777 to no avail. I've always tried it with and without the trailing slash in my.cnf, though I don't think that makes a difference. The appArmor profiles are defined as such:

/data/mysql/ r,
/data/mysql/** rwk,
/data/tmp/ r,
/data/tmp/** rwk,

I additionally tried rebooting the instance to no avail. There are no errors, and the logs note nothing substantial. There must be some variable I'm missing - any linux/mysql gurus know what could be causing this? Thanks!