Short Answer:
The variant probably pre-dates the 5th century
Long Answer:
This is a variant with a long and interesting pedigree. Although we do not know the exact date of the variant--referring to the prophets instead of to Isaiah--it is indeed found in texts prior to the Textus Receptus.
The 1550 Textus Receptus of Stephanus (see here) references "prophets" rather than Isaiah:
ως γεγραπται εν τοις προφηταις ιδου εγω αποστελλω τον αγγελον μου προ
προσωπου σου ος κατασκευασει την οδον σου εμπροσθεν σου
As noted by Tony Chan, the "Isaiah" reading is generally preferred by textual critics because of its presence in very early Alexandrian manuscripts (e.g. Codex Sinaiticus, Codex Vaticanus, the relevant manuscript page of which can be viewed here).
Codex Alexandrinus is a very early example of the "prophets" variant in the Byzantine text type; showing that the variant existed as early as the 5th Century. Wikipedia has a nice summary of which reading is found in some of the earliest manuscripts.
The Alexandrian vs Byzantine text debate is a bigger issue than I'll try to tackle in this post, but the Textus Receptus relied heavily on Byzantine texts.
I'll cite just one reason--from textual criticism--for favoring the "Isaiah" reading.
It is generally held that scribal emendation--especially a change that sticks--is more likely to smooth out a hard reading than the other way around. We shouldn't use that as a universal generalization (remember that line: "all generalizations are false!" =) ), but it is a frequent and relied-upon tool in the study of ancient manuscripts.
Since the OT quotation is not simply a quotation from Isaiah, but in fact a merger of a quote from Malachi with a quote from Isaiah, a scribe would be more likely to update the reference to Isaiah (because it's quoting multiple prophets, not just Isaiah), than to take a reference to prophets and replace it with Isaiah specifically. So the theory goes that somewhere in the family tree of the Byzantine manuscripts a scribe thought it would be helpful to replace "Isaiah" with "prophets", since both Isaiah and Malachi were quoted.
