Hello Nischal. Manish is pointing in the right direction. SailPoint does document the expected delay. The Managing Snapshots page says changes to access objects can take up to 6 hours to appear in Identity Graph. I would treat that as a propagation window, not a fixed refresh schedule.
Since you mentioned the identity was terminated a while ago, I would check when that entitlement actually disappeared from ISC. If Search and the Identity page have shown it as removed for well over six hours, but Identity Graph still shows the relationship, I would open a Support case. I would include the identity ID, entitlement ID or name, approximate removal time, and a JSON export from Identity Graph if possible.
For the data-source question, Identity Graph uses the identity and access data available in ISC and SailPoint describes it as built directly on Atlas. I could not find anything public confirming it uses the same refresh pipeline as AI or AIC, so I would not assume they refresh together.