I have a tundra growing right now and its by far the best strain I have every grown (even being an auto!). All the seeds autoflowered just fine (I got 3 seeds) and turned out great, you must have got the unlucky seed that didn't get the autoflowering genetics! Has it started flowering since you put them in 12/12? I bet it turns out to be a great strain anyway!
I'm excited cuz I got an auto-himalayan blue diesel and diesel ryder males that are about to release pollen and I'm gonna cross it with some of the smaller tundra nugs to get a bunch of awesome auto seeds. (I think the blue diesel and tundra will be a GREAT combo.)
And please don't give up on the tundra yet if you got any more seeds! Its some of the best smelling bud you will ever grow, really fruity and pungent, you literally just wanna stuff your nose in it! Good luck!