Disclaimer: I had no chance to test it so I don't guarantee all information on this post are correct.
UPDATE: Please read comments below this article for further information and great Martin's explanation of real MTU behavior. Thanks Martin and Kaloyan for your comments.
MTU 12000 in configuration was not reflecting real hardware MTU of underlaying chipset, after upgrade to 9.10 it's just adjusted to reflect real hardware MTU. Tested on S4048 9.10(0.1). When you boot into 9.10 you can see log messages saying that configuration is adjusted to reflect real maximum hardware MTU.
Also in configuration
ethswitch1(conf-if-te-1/47)#mtu ? <594-12000> Interface MTU (default = 1554, hardware supported maximum = 9216) ethswitch1(conf-if-te-1/47)#mtu 594-12000>