When I use the forum search and search for topics matching a certain string and change the sort order to post count, it still sorts by date.
Here's an example query: https://pinside.com/pinball/forum/search?s=1&what=topics&q=getaway&topic-machine=&pinsider=&age_days=&subforum=0&sort_by=postcount&sort_order=DESC#results
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Your browser/os information:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:46.0) Gecko/20100101 Firefox/46.0 (this will help us make sense of your bug report)