Task dates are incorrect in backward scheduling mode

[arcady-zherdev]

Reported here: https://forum.bryntum.com/viewtopic.php?f=52&t=24289&p=120619

Steps to reproduce:

1. unzip the attached launch-saas.json file and put it to examples/_datasets folder
2. open advanced demo
3. expand all tasks

Result: Project Management task (the last one) has both start and end date set to 1st of October. Yet it has these parameters

"startDate": "2022-05-06",
"endDate": "2022-10-01",
"duration": 106,
"manuallyScheduled": true,

[canonic-epicure]

This is a duplicate of the #6406