Topic search is only available at repository level currently: https://help.github.com/en/articles/searching-topics
In terms of the flow you are looking at you may need to perform a couple of searches:
-Firstly use topic search https://help.github.com/en/articles/searching-for-repositories#search-by-topic to isolate the repositories that you want to look at
-Secondly use issue search within the repository that is returned to isolate the issues that you are looking for.
You could theoretically use the Search API to automate this: https://developer.github.com/v3/search/ this could first search for the repositories with a given topic, then iterate through the ones that you are interested in and perform a search for issues with those criteria.
However, be aware of rate limiting in this instance: https://developer.github.com/v3/#rate-limiting if you are performing a number of searches.
I hope this helps
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