Wipe unsuccessful with encrypted system partition

Hello,

I am testing the wipe command on a phone with TWRP + encrypted system partition + cyanogenmod 11.
I have sent myself the SMS command ‘xxxx WIPE’ and seen the phone reboot (after some time).
After the reboot I am brought to TWRP recovery and asked for my encryption password (to mount the system partition). To simulate an actual theft event I hit ‘cancel’ and rebooted the phone. I now have a half-wiped phone (sd is empty) that still retains all my accounts and passwords.
Could you please clarify if this is normal behavior? I would have expected to find a fully ‘new’ phone after the reboot but perhaps this is a limitation of how wipes are done when / is encrypted?
I guess it would still be ok-ish since the thief would not have my system partition password and therefore should not be able to access my data, but still… I would be happier with a full wipe!

Thanks,
Andrea