Topics API returned on few percentage of traffic

[dediha]

Hi,
We have very few percentage of topics returned from the browsingTopics() api,
Is it related to calling it from the root document(SSP side) when referring to document.browsingTopics(),
I saw in the spec referring to only iframe or fetch: https://patcg-individual-drafts.github.io/topics/#introduction-header
Or should this be the expected outcome?
Might this be related to the new taxonomy?
Would appreciate your advice