This doesn't seem to be a recent "Protestant" issue, as the early church itself used a similar term in an ambiguous way: the "rule of faith/truth" (regula fidei/veritatis). Here's B. Hägglund's summary:
The regula fulfils the function of being a fundamentum of the doctrinal tradition through the mediation (Vermittlung) of the holy scripture. We can perceive the reality of the revelation, the facts of salvation history only through the witness of the prophets and the apostles, through the writings of the Old and New Testaments. This witness must be interpreted and expounded again and again, but also recapitulated (zusammengefasst) and literally reproduced. In the process, however, the regula itself, the truth to which the scripture witnesses, maintains its position as an unchanging foundation. It is not a coincidence that the Greek word for rule, κανων, became more and more a fixed designation for the holy scripture. The original witness is not only “canonical” because it is endowed with the authority of the prophets and apostles, but also because it is a bearer (Träger) of the revelation, a mediator of the reality of salvation." (My translation; for the full context go here).
(1) the present preaching of the Church as related to the word of Scripture and referred to it as a norm; (2) the witness of Apostles and Prophets as contained in the canon of Scripture to the Word of God; and (3) the Word of God itself as revelation. (Diem, Dogmatics, pp. 57-58).