Are there more than 1 PM Work Orders that got canceled? Generally, it should not be an issue in reverting the due dates when done individually. May be this has happened and you might need a fix as per given APAR.
https://www.ibm.com/support/pages/apar/IV24986
For now, you can generate WO manually with ignoring the frequency for the PM to happen and the adjust the next due date from Frequency tab to date when work order should generate next as per original schedule.
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Prashant Sharma
Sedin Technologies
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