The issue maybe persists with the file caching adapter too or any other. Using Cm_RedisCache for the standard magento cache layers does not produce data leakage. To reproduce activate FPC and cache only one page. Look at the cache entries from FPC. Go to the cache management and delete FPC cache. Look back at the cache entries left from FPC - you will find entries of the tags used for the cached data. They should be cleared too. Look at this image showing the database storing the FPC data after caching one page and clearing the FPC from the admin panel.
The issue maybe persists with the file caching adapter too or any other. Using Cm_RedisCache for the standard magento cache layers does not produce data leakage. To reproduce activate FPC and cache only one page. Look at the cache entries from FPC. Go to the cache management and delete FPC cache. Look back at the cache entries left from FPC - you will find entries of the tags used for the cached data. They should be cleared too. Look at this image showing the database storing the FPC data after caching one page and clearing the FPC from the admin panel.
https://ibb.co/iuLM8d