(message "%s" M) is the usual way to output a message M without 
formatting it, and something like this is needed anyway if M might 
contain %. Accordingly, I installed the attached patch into the emacs-25 
branch and merged the result to master. I had already adjusted some 
other instances of calls to ‘message’ (to fix problems with % as well as 
with quotes) but missed this call; sorry about that.

The behavior of ‘message’ is documented in the manual, in doc strings, 
and the quoting change in Emacs 25 is documented in etc/NEWS, so it is 
not a bug per se.