Hi,
I think you're right, it should be UD date + inspection interval = new next inspection date. Couple things to check in addition to what Arijit Banerjee mentioned...
1. Check the material change history - was the inspection interval 30 days when the lot was created / UD done?
2. Check to see if there is some custom logic in exit QEVA0003 (EXIT_SAPMQEVA_003)
Good luck!