This phenomenon involving ὅτι is called “attraction.”
On the Greek word ὅτι, Thayer wrote,1
Noteworthy is the attraction, not uncommon, by which the noun that would naturally be the subject of the subjoined clause, is attracted by the verb of the principal clause and becomes its object [cf. W. § 66, 5; B § 151, 1 a.]; as, οἴδατε τὴν οἰκίαν Στεφανᾶ, ὅτι ἐστὶν ἀπαρχή. for οἴδατε, ὅτι ἡ οἰκία Στ. κτλ., 1 Co. 16:15; also after εἰδέναι and ἰδεῖν, Mk. 12:34; 1 Th. 2:1; so after other verbs of knowing, declaring, etc.: Mt. 25:24; Jn. 9:8; Acts 3:10; 9:20; 1 Co. 3:20; 2 Th. 2:4; Rev. 17:8, etc.; ὃν ὑμεῖς λέγετε ὅτι θεὸς ὑμῶν ἐστι, for περὶ οὗ (cf. Lk. 21:5) ὑμεῖς λέγετε ὅτι, Jn. 8:54.
Very oft. the subj. of the ὅτι-clause is drawn into the main clause, and becomes the object of the latter: ἐπεγίνωσκον αὐτοὺς ὅτι (=ὅτι αὐτοὶ) σὺν τῷ Ἰησοῦ ἦσαν Ac 4:13. οἴδατε τὴν οἰκίαν Στεφανᾶ ὅτι (=ὅτι ἡ οἰκία Σ.) ἐστὶν ἀπαρχή 1 Cor 16:15. Cp. Mt 25:24; Mk 12:34; J 8:54; 9:8; Ac 3:10; 1 Cor 3:20 (Ps 93:11); 1 Th 2:1; Rv 17:8. Somet. the subj. is repeated by a demonstrative pron. in the ὅτι-clause: ἐκήρυσσεν τὸν Ἰησοῦν ὅτι οὗτός ἐστιν ὁ υἱὸς τοῦ θεοῦ Ac 9:20.—Pass. εἰ Χριστὸς κηρύσσεται ὅτι ἐκ νεκρῶν ἐγήγερται (=εἰ κηρύσσεται ὅτι Χρ. ἐκ νεκ. ἐγ.) 1 Cor 15:12.
With respect to Mark 12:34, «ἰδὼν αὐτὸν ὅτι νουνεχῶς ἀπεκρίθη...» would be understood as «ἰδὼν ὅτι νουνεχῶς ἀπεκρίθη αὐτὸς...».
More information can be found in Buttman3 and Winer4 in the references below.
As far as the textual critical aspect is concerned, just as many manuscripts appear to lack it as have it. According to Constantin Tischendorf,5
Porter and Pitts describe the significance of the brackets,6
Square brackets () signify that the enclosed reading is one that the committee could not reach a certain decision on so they include the text in brackets to indicate doubt.
Arndt, William; Bauer, Walter; Danker, Frederick William. A Greek-English Lexicon of the New Testament and Other Early Christian Literature. 3rd ed. Chicago: U of Chicago P, 2000.
Buttmann, Alexander. A Grammar of the New Testament Greek. Trans. Thayer, Joseph Henry. Andover: Draper, 1873.
Pitts, Andrew W.; Porter, Stanley E. Fundamentals of New Testament Textual Criticism. Grand Rapids: Eerdmans, 2015.
Tischendorf, Constantin. Novum Testamentum Graece. Vol. 1. Lipsiae: Giesecke, 1869.
Wilke, Christian Gottlob. A Greek-English Lexicon of the New Testament: Being Grimm Wilke’s Clavis Novi Testamenti. Trans. Thayer, Joseph Henry.Ed. Grimm, Carl Ludwig Wilibald. Rev. ed. New York: American Book, 1889.
Winer, George Benedikt. A Grammar of the Idiom of the New Testament. 7th ed. Andover: Draper, 1892.
1 p. 459, §3
2 p. 732, 1, f
3 p. 376, §151
4 p. 626, §66, 5
5 Vol. 1, p. 354
6 p. 150