Sorry my bad, i was looking at the wrong interfaces. Now my best guess is that in 14.1X53-D10 new link protection config option was added in for newer EX models, namely EX2300, EX3400, EX4300 (and EX4600??). See:
http://www.juniper.net/techpubs/en_US/junos/topics/task/configuration/lacp-link-protection-cli.html
Now 15.1 is a release that supports whole EX family, both legacy (EX2200/3300/4200/45xx) as well as new. I've got to believe some how [in the code] there is a mix-up between the 2 different procedures and that is why your link protection may not be working properly when you move to 15.1. Just FYI this is my best quess.
I am wondering in the meantime if you could use active-active AE config, and then open a case with TAC on your 15.1 issue?
I am just sort of curious why you are using primary/back-up for your AE, as I had never seen this type of implemenation used previously, and was wondering what your reason for using this is? Yes it is an option that regardless should work, but just curious.
Hopefully this response makes more sense to you.