When we cancel days and add new ones, customers can still click "resume" in the app, resulting in more days than the total subscription days. What’s the issue?

The issue here was that you paused the selected days (example: 4 thursdays), then instead of clicking on “resume” to add 4 new days (example: 4 saturdays), you clicked on “add extra days”.
This means that there are still 4 paused days, and that’s why the customers could add them.