From IBM support

Case 1: User A has modify rights to User B's Mail, Calendar, To Do, Contacts
 > When User A opens the calendar of User B, it will open on the view where it was last opened

Case 2: User B has modify rights to User B's Calendar, To Do and Contacts
> When User A opens the calendar of the User B, it will open on the One Week view, regardless of the last view it was opened

The difference between the two is the Access to the mail file. We can check this in the Access Control List (File > Application > Access Control). Case 1 creates at least AUTHOR access while the Case 2 have No Access. Thus User A should have at least  Author access to the mail file of User B for this to work. This is working as designed.

In line with this, I would like to inform you that I have created  #LO84670 with SPR #ADON9VNTWF for enhancement request regarding the issue.