At Complete duration units change every time project is scheduled.

The at complete is an estimate of the duration at completion time for the selected activity. (At Complete Duration = Actual Duration + Remaining Duration). If the selected activity is in progress, type a new at complete estimate. For the most part, Primavera will only keep the Original Duration fixed in fixed duration & units activities. It’s basically allowing the user to say that the remaining or at complete could change. Actuals, will come in through timesheets.

For the most part, yes, the at complete should remain close to the original duration. It will be the same when the activity is created, but the remaining and at complete will change as the project progresses. Part of the update process is to update remaining units which would directly impact the at complete. Every week actuals are changing, PM updates the remaining (usually reduces them) and at complete will sync with Orig Duration.

In Activity Details, you can adjust any activity information, such as Expected Finish or Remaining Duration, that did not progress according to plan. Since you actuals pulling in, that’s the only field that will update once actuals are applied.

There is an instance that Actual Duration + Remaining Duration Does Not Equal At Completion Duration.
When the Remaining Early Start date of an in-progress activity is delayed beyond the first available work time after the data date (prior Sunday of current work week), then the formula Actual Duration + Remaining
Duration will not take the time between the Data Date and the Remaining Early Start into account. This is why there can be a difference between Actual Duration + Remaining Duration and the At Complete Duration.