The isCached() method was always checking the actual cache, and returning true if the cached existed. The actual displayCache() method won't use the cache if debug is on, however. So turning on debug would cause any code relying on isCached() to fail to generate the content and then displayCache() would recreate the cache with missing data.
I modified isCached() to always return false is debug mode is enabled. This is fixed in SVN and will be in the 0.73 release.