Depending on the configuration of the App, your tasks might get updated or recalculated during the synchronization, for example, once the scope is defined. By default, the App requires two points on the timeline (in other words two dates) to generate a taskbar and determine the start and endpoints. Those points in time are the task's "Start Date" and "End date" and are stored as "Start Date" and "End date" custom fields by default.
If a Task with no Start and/or End Dates is added to the scope of a Box, the App will pick the Original Estimate value to calculate the duration and set the task's Start date to the Creation date.
Following this logic, if no Original Estimate value is available the app will again use the creation date as the Start Date and when the End Date is empty, then it picks the resolution date (if the issue was Resolved) instead. This usually means that a Task will be set as a 1-day duration starting on the Creation date. With the Original Estimate mapped as the Start or End Dates, it will represent the duration of a Task.
As a consequence, in such a scenario, the period mode will be applied to tasks with empty date fields to avoid data corruption when the tasks are structured automatically using presets.
In the case of Sprint and Version tasks synchronization, the "Auto top-down" is used by default.
Read more about the scheduling and synchronization settings in the Scheduling section.