We found the issue that is causing this to happen. Starting with service pack 18, the lodash clientlib has been removed from AEM. We had a piece of code with a dependency on lodash, and without it there, the search could not function properly.
I saw that other thread before posting my question and based on the comments in there, it appears this is an issue with the product, since someone was able to reproduce the issue on a vanilla instance running service pack 15.
After upgrading to service pack 18, the search within the Author Assets section of our AEM does not function properly. There are 3 main issues:1. The filters section on the left side of the search results does not appear or function. 2. The number of results on the page shows incorrectly as -1 of -...