Employee was permanently transferred from A to B. The transfer went okay, however, all booked holiday wasn't transferred and was removed from the calendar. Why?

When an employee is permanently transferred to another site, all their booked holiday in the future gets removed from the system following the transfer.

When an employee moves to another site on a permanent basis the system cancels any scheduled holiday and future requests. This is because the holiday was approved by the old manager at the old site where the busy period may differ when compared to the new site. Therefore it is important that the new manager reviews the holiday requests to ensure that their site is not understaffed. The current procedure is that the employee has to request the holiday again when at the new site to ensure that the holidays requested are in line with the new site's needs. 

Please see here the article that explains how an employee can request a holiday through the S4 connect (employee app):

How do I request Holiday through S4 Connect?