I've been working on this prison system and use Data Persistence to save the amount of seconds they still have left. I've noticed when the value is 0, it prints 'save', but when I load it after rejoining, it appears it never saved at all. First script is the Saving one; Second is loading.
*Script Removed
I am assuming ROBLOX just wants to save data and Data Persistence requests by doing this, but they don't save the number 0. The best thing you could probably do is save the number as a -1 and when the player rejoins have the script load their jail time as 0 seconds.