When enabled, the auto-approve service will approve or deny time off requests based on preconfigured rules for blackout dates, brownout dates, and minimum request interval. It will also check for enough accrued hours (see extra credit) and whether the dates fall within a calendar partition (if the agent's rule source is calendar partition rules).
If auto-approve isn't working, check the Services settings.
Settings > Application settings > Administrative settings > Services > Auto-Approve Service

Basic options tab
- Is the service turned on? Check if Service enabled? is set to Yes.
- What is the setting for process delay? Default is 10 minutes. With the increased number of rules in the system, it takes longer to run through all of the rules.
- How far in advance are the requests? Check the Number of days in advance to consider event requests and adjust to cover the dates of the time off requests. Default is 90 days.
Advance options tab
- Are single-day event permissions days checked? Check the days on which you want the service to run. If a day is not checked, the service will not auto-approve requests for that day of the week.
- Is it a multi-day event? Check the box for the service to auto-approve multi-day time off requests. This applies when an agent selects Yes, all the dates in the range must be approved together for the question Should all dates in the date range be approved together? on the Request Time Off form. This does not apply to 'recurring time off event' requests with multiple days.
Extra credit
Why did the legacy auto-approve system approve time off when the agent didn't have enough accrued hours?
If the agent is assigned to Legacy AutoApprove Rules, check to make sure the Agent Accrued Hours Rule is set to Yes for Enforce this time off rule? and Default Validate at runtime is set to Yes.
Plan > All things time off > Time off settings > Time off rule settings.