I just tested the GPU setting on a new image and it worked right after reboot.
What Raspberry Pi model do you use?
The version 2.26 should still be based on Raspbian wheezy, another Debian source.
However, my installation with 2.44 based on Raspbian Jessie lite saved the change to 16 right away after reboot.
Could you share the complete debug info of your Raspberry Settings?