try above query, it should work. You already applied a filter and you no need to declare it again to filter out the results.
still does not work.
In pact your conclusion of declaring filter thing is not true as well
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https://developers.jivesoftware.com/api/v3/cloud/rest/ContentService.html#getContents(List<String>, String, int, int, String, boolean, boolean)
There is no filter after.
You can use creationDate or modificationDate, e.g.
Maybe your request was not clear- the search endpoint has an after filter, but not the content endpoint. And you are using the contents endpoint here.