Poor Tsutomu Mizushima. Fan reaction to Girls und Panzer was very positive. Blu-ray sales were great. He just couldn't get the darn show produced on schedule while maintaining his standards. After needing a 5.5 "Shōkai Shimasu!" ("Let Me Introduce") episode November 13th, he's had to slate a "Shōkai Shimasu 2!" for Christmas Eve.
As a consequence, the show could not complete its planned twelve episodes in time for the allotted TV broadcast slot. With all luck, episode 12 is not going to be the last we see of Anglerfish Team and company, but it was at least supposed to bring the story to some sort of punctuating climax.
Well, the plan is to finish episodes 11 and 12, fix up episodes like 10 for the Blu-ray release, and hopefully air the end of the series in March.