On line 28 of the template/feed-json.php file, you have incorrectly set the cache key. Using the incrementer (which uses the time() function) to set the key means that every 12 hours (or whatever threshold you've set), the cache key will be different, causing the options table to fill, and the cache to miss all the time. Removing the incrementer and just using the query makes it work. Here is the code to fix that version.
$cache_key = 'fj-query-' . md5( http_build_query( $query_array ) );
In the future, please mention that you removed the cache function when pushing new versions. You highlighted that caching was in 1.0.7, but then published nothing in a changelog (at least, I couldn't find it) about the issue or that caching was removed in v1.0.8.