I also had the issue flashing from the november WRT3200acm build to this December 18th build. The final sha256 check in the web interface did not match the download check and the flash aborts. I succeeded by rebooting into the previous build on the alternate partition and in this older version upon flashing the sha256 check matched the download file check. The flashing process went through flawlessly.
I think there is something amiss in the build from November with regards to the flashing process.
I did initiate a flash to the stock open WRT 18.06.1 build and upon viewing the sha256 check in the WebUI, it appears this problem does not exist in the December 18th build so we should be good for next time.