mo·tif (m-tf) n.
mo·tif n.
1.
a. A recurrent thematic element in an artistic or literary work.
b. A dominant theme or central idea.
2. Music A short rhythmic or melodic passage that is repeated or evoked in various parts of a composition.
3. A repeated figure or design in architecture or decoration.


Since motif in music is described by def. 2 as repeated or repeatedly evoked, I would say the answer to your question is a stable apex tone; the deflection to the A fails to achieve that stable tone.

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A great many people will think they are thinking when they are merely rearranging their prejudices. ~ William James ~

I agree with the point you raise in your question, i.e. that this sentence could be read either way.

I don't think there is any way to determine which of the opposing meanings is intended as the sentence is written, (that the stable apex tone occurs in the basic motif, or that the failure to achieve a stable apex tone occurs in the basic motif), except perhaps I would be a tiny bit more inclined to think that the phrase which is placed nearest to the verb is the one meant, i.e. that the stable apex tone is what occurs in the basic motif.

However, with this unclear sentence construction I would not have very high confidence in that suggestion. I think if it was just changed as follows, that it would be clear:

The downward deflection to the A represents a failure to achieve a stable apex tone, such as occurs in the basic motif.

With this construction, I think it is clear that such as occurs ... refers to the stable apex tone, and not to the failure to achieve it.

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I would live all my life in nonchalance and insouciance, Were it not for making a living, which is rather a nouciance. Ogden Nash