I am still able to reproduce this issue with gimp 2.8.14-1ubuntu1 on Ubuntu 15.04.
However, I also tried with 2.8.14-1+b1 on Debian Sid, where the menu worked as expected after pinning it by clicking on the dotted line. So it looks like the problem is Ubuntu-specific. (I didn't see any error messages or other output when running gimp in a terminal related to this.)
I am still able to reproduce this issue with gimp 2.8.14-1ubuntu1 on Ubuntu 15.04.
However, I also tried with 2.8.14-1+b1 on Debian Sid, where the menu worked as expected after pinning it by clicking on the dotted line. So it looks like the problem is Ubuntu-specific. (I didn't see any error messages or other output when running gimp in a terminal related to this.)