Before topic matching we used random scoring to get a random subset of tasks. Topic matching itself is a kind of sorting, which might be incompatible with this. Since we want to avoid giving the same task to all users who have the same task type / topic configuration, we need to figure out another way to make the task list non-deterministic.
In the short term, we can just shuffle the search results we get. That might be fine in the longer term too, or there might be another solution which makes ElasticSearch combine multiple score types. See the discussion starting at T240512#5773074 for more details.