Are Making Tax Digital quarterly updates cumulative?

Yes, and this is the part almost nobody expects. Each update covers from the start of the tax year to the end of that period, not just the three months since the last one.

So the November update repeats April to July and adds August to October. The February one covers April to January. The final one covers the whole year.

The practical effect is that a mistake does not stay buried in one quarter. You correct it in the next update and the corrected figure carries through, which is genuinely easier than amending something.

It also means a quarter you thought you had finished with is never quite finished with. HMRC sets this out on gov.uk.