Our team just discovered this issue too, specifically when using the advanced-custom-fields plugin.
$found is used when there is a possibility that your cache object stores boolean values, particularly false. If a cache hit from wp_cache_get() actually returns a value of false then you cannot reliably determine a cache hit/miss this way. wp_cache_get() would return false for both a cache hit and miss in this situation.
Instead of checking the value of wp_cache_get() to determine cache hit/miss, you can pass $found by reference to determine success. $found will return true/false for cache hit/miss I believe.
I haven’t tested this yet but this fork looks promising: https://github.com/voceconnect/memcached. It appears to address the issue.
