### Steps to reproduce

1. Begin of the insertionCreate two work packages following each other without gap

1. Saturday and Sunday are all non-working days

2. predecessor ends onEnd of the insertion Begin of the deletion Go toEnd of the deletion a Begin of the insertionFriday (2025-04-04)

3. successorEnd of the insertion Begin of the deletion WP in automatic scheduling mode where start dateEnd of the deletion is Begin of the insertionautomatically scheduled, starts on next Monday (2025-04-07, derived) and ends on next Wednesday (2025-04-09)

End of the insertion Begin of the deletion set by a predecessor:

End of the deletion 2. Open the date picker Begin of the insertionfor successorEnd of the insertion

3. Begin of the insertionUncheck "Working days only" and change dueEnd of the insertion Begin of the deletion Add a finishEnd of the deletion date Begin of the insertionto Thursday (2025-04-10)End of the insertion Begin of the deletion by clicking on the calendarEnd of the deletion

4. Save Begin of the deletion

5. Reopen the date picker

6. Change the finish date manually

7. SaveEnd of the deletion


### What is the buggy behaviour?

* Begin of the insertionWhen unchecking "Working days only", startEnd of the insertion Begin of the deletion FinishEnd of the deletion date Begin of the insertionshould have changed to Saturday (2025-04-05) in the date picker, as that's the soonest start date if it's ignoring non-working days

* After saving, the finish date was set to Tuesday (2025-04-08) whichEnd of the insertion is Begin of the insertionwrong: Thursday (2025-04-10) was explicitly chosen to preserve the durationEnd of the insertion Begin of the deletion saved but not updated End of the deletion


### What is the expected behaviour?

1. Begin of the insertionStartEnd of the insertion Begin of the deletion FinishEnd of the deletion date is Begin of the insertionupdated in the date picker when "Working days only" is toggled if appropriate

2. The selected finish date is usedEnd of the insertion Begin of the deletion savedEnd of the deletion and Begin of the insertionduration isEnd of the insertion updated


Begin of the insertion### Why this bug?

It seems to happen when modifying both due date and "Working days only" on an automatically scheduled work package.

It happens only when changing due date and "Working days only", and when changing it modifies the start date. 

When saving, it does not correctly take "Working days only" value into account to recompute the soonest start date, so the start date does not change and due date is set to the given date. Then, because due date is changed, a rescheduling occurs. The rescheduling correctly computes the correct start date, so it moves to the correct start date, keeping the duration previously computed, which shifts the due date by the same amount of days as the start date.

End of the insertion ### Screenshots and other files

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/414921/content">

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### Environment information

**OpenProject version**

Core build

[204a3ec0d2ad3ace739ef1f49134ee14bfb609d0](https://github.com/opf/openproject/commits/204a3ec0d2ad3ace739ef1f49134ee14bfb609d0)

Product version

[0e381ca55ef56dbc8dc3bd2e904af5ace7dd303b](https://github.com/opf/saas-openproject/commits/0e381ca55ef56dbc8dc3bd2e904af5ace7dd303b)

Core version

OpenProject 15.5.0

**Browser**

* [ ] Chrome
* [x] Firefox 136.4.0
* [ ] Safari
* [ ] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [x] Windows 11Pro
* [ ] Mac OS X
* [ ] Mobile iOS
* [ ] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)