Many heretical texts were not preserved though it's more likely they were simply copied less due to being less popular. What's known of them is preserved in the church fathers discussing them and arguing against them. But aside from the fact there's no positive evidence that heretical texts were deliberately destroyed, the fact that the church fathers quote them and make arguments against them in surviving documents shows there's was no campaign to eradicate discussion of them nor hide the fact that they ever existed.

Which is what makes it even less likely that there was some great debate over what the text of, say, Matthews gospel was that was then settled and somehow covered up. There's no evidence that this took place at all.

The original author of (for example) Mathew was not the apostle Matthew, it was written somewhere around 80-100. But the fact that a single version was widespread in the second century and even churches far apart are quoting the same version and evidence for any debate of its content is lacking that shows the text is most likely stable from very shortly after it was written. The same goes for the other gospels, Paul's undisputed letters and most other NT texts.

"Heresies" were entire other books, not different versions of the canonical texts. There were examples of short forms of gospels being used (Marcion's Luke) but not entire alternate versions.

The simplest explanation for this is that gospels were produced by a sponsoring church (in Matthews case, Antioch) and then accepted by other churches and rapidly copied widely. There is no evidence later churches substantially rewrote it and some sort of battle ensued.

We know from how church fathers talked about other disagreements that there would have been no issue preserving such a dispute had it happened.