Hi Ryoske.
That idealy shouldnt happen, unless you are deleting the PRs created previosuly. Are you deleting the PRs with past delivery dates?
Check if there was any change in the MRP parameter recently, leading to deletion of open PRs or some user has manually deleted them.
If this is not the case, you raise this to SAP. This needs thorough analysis in your system
Regards
Vivek Singh