I made a quick screencast, where you see that after changing the syntax, closing the file and opening that same file again, the syntax setting has reverted.

http://recordit.co/FOEk27mggq

Even though the fileType is reported as text.html.erb.rails (see screenshot)

My Global.tmProperties:

https://gist.github.com/koenpunt/a57e209ddb0ab0af119b59cc86bc90af

And my .tm_properties in my home folder: 

https://gist.github.com/koenpunt/02dd3ab3670586216d40a9dacf42d3c3

Koen