User issued temporary ban, it was made permanent

On a subreddit I moderate, a user was issued a 3-day ban for breaking a subreddit rule. After 4 days, they reached out to inquire as their ban was still in effect; when checking the mod log, I could see "Banned for 3 days," yet when hovering over the Unban button, it showed that it was actually a permanent ban. None of our mods know why this happened; there is no record of a permanent ban in the log. Anyone else had this happen? If so, do we know why this happens/how to avoid it?