I was using addtask foo before bar after x after y which I think is incorrect syntax, although bitbake didn't warn, but instead silently zapped the dependency on "after x". Correct syntax would be addtask foo before bar after x y (only one 'after'). I suggest either bitbake should handle the first version as well, or it should give an error when parsing.
Should results in an error.
Fixed on master branch: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=82300691b045729dd35aec6878b4c199bea835fc