Sometimes Roman Catholics will argue using formal sufficiency without admitting to it.
William Whitaker (1547-1595):
Indeed all the papists in their books, when they seek to prove any thing, boast everywhere that they can bring arguments against us from the most luminous, plain, clear and manifest testimonies of Scripture . . . For in every dispute their common phrases are,—This is clear,—This is plain,—This is manifest in the scriptures, and such like. Surely when they speak thus, they ignorantly and unawares confess the perspicuity of the scriptures even in the greatest questions and controversies.
See: A Disputation on Holy Scripture Against the Papists, Especially Bellarmine and Stapleton, trans. and ed. William Fitzgerald (Cambridge: The University Press, reprinted 1849), p. 401. See also, this link.
Keep in mind as well, some Romanists will redefine Biblical material sufficiency to mean "Every true doctrine must be in harmony with it. That is not the same as saying that every doctrine must have explicit biblical proof."
In both instances, one needs to be aware of the double standard and redefinition of terms. You'll save yourself a lot of time in dialogue by exposing these presuppositions.