In Approve Leave request V3 via My Inbox, the overlap calendar is only shown when the below criteria are met:
Employee A is manager, Employee C and D are reporting to him.
If employee C and D applied leave on same day say 29.11.2020, In Approve leave Request when manager tries to approve employee C or D leave request, he can see employee C and D have applied leave on same day in the details screen of the leave request with overlap calendar.
In other scenarios it will not be showed.
The way the colleagues and the manager are determined is via the code implemented in Badi HCMFAB_B_COMMON methods
A default fallback class of this code is delivered. If you have implemented the badi, please make sure that the BADI is retrieving the data.