Mt 21:23 reads in NA:
Καὶ ἐλθόντος αὐτοῦ εἰς τὸ ἱερὸν προσῆλθον αὐτῷ διδάσκοντι οἱ ἀρχιερεῖς καὶ οἱ πρεσβύτεροι τοῦ λαοῦ λέγοντες· ἐν ποίᾳ ἐξουσίᾳ ταῦτα ποιεῖς; καὶ τίς σοι ἔδωκεν τὴν ἐξουσίαν ταύτην;
For the omission are noted in SQE (not NA): 7, it, Sy-S, Sy-C, Orpt
Fortunately ms 7 can be found in the VMR: GA 7 in the VMR
The text in question is on image 1110.
Here is the relevant part:
|Mt 21:23 in Codex 7|
As one can see, the word is clearly present, although we have a word-order variant:
Καὶ ἐλθόντι αὐτῷ εἰς τὸ ἱερὸν διδάσκοντι προσῆλθον αὐτῷ οἱ πρεσβύτεροι τοῦ λαοῦ λέγοντες·
Streeter ("Four Gospels") called attention to this omission. But he was under the wrong impression from Tischendorf that D supports the omission.This is probably the reason why the variant had been added in SQE (Western non-interpolation).
The omission in the versions is probably for stylistic reasons.
Note also that διδάσκοντι / διδάσκοντος is one of the so called Minor Agreements of Mt and Lk against Mk.