PR Release strategy is reset on every MRP run

In our plant we have a 4 level release strategy for particular PR's The 4 levels are successive for PR values. PR created via workorder retain the release on all levels endlessly, meaning that releaser a can release on day 1 and successive releaser b can release on day 14. When PR are created via MRP run, above release strategy applies. MRP run’s daily. When the PR is not released and a PO is created before the next MRP run the releases are all reset and the releases have to do is all over again. What can be the cause of this behavior?