Unfortunately, I can't explain why it works in the beginning and then stops. You could check the following approach.
- Create a Today column (format its output as date and time):
2. And in the DaysOpen
column (I formatted it as a number), use this formula:
Perhaps, it will help. If not, I would advise you to contact Microsoft support since it is pure SharePoint functionality. I would like only to draw your attention that the calculated column is required only for displaying the number of days. Scheduler tasks don't need it (see below).
Regardless of the calculated column, you can create a task that will use functions in its condition to define whether it is time to close tickets.
AddDays([Ticket.Created], 10) - adds 10 days to the date from the Created column.
Date(AddDays([Ticket.Created], 10)) - takes the date part without time from the previous action.