IBP Operator Time Selections Degrading / Changing by Themselves

Hi experts. We are finding that the operator time selections of stable operators that have been running correctly for quite a while (6 months or more) suddenly change to something unwanted. For example, an operator that has an offset, suddenly the offset has changed. It has happened 3 or 4 times over the last year or so, and we have not been able to see any pattern or consistency. Has anyone else had something similar happen and or any thoughts on causes? The only hypothesis we can think of is that something in the quarterly upgrades could be a cause, but this is only a guess, based on that we have not made any changes to our design or config over this period so quarterly upgrades appear to be the only "change" that could be causing the issue.