RFC 4791 6.2.1: »CALDAV:calendar-home-set Property
Typically, users will group all the calendar collections that they own under a common collection. This property specifies the URL of collections that are either calendar collections or ordinary collections that have child or descendant calendar collections owned by the principal.«
So, I think this is a matter of interpretation. What is “the URL of calendar collections”? “The URL” is singular, “calendar collections” are plural. Until I have found/heard good arguments, I will interpret it as “URL that contains calendar collections (but not: is a calendar collection”) and then DAVdroids behaviour is correct.
What I have missed is that the home-set can contain nested collections and only the last level must be a calendar collection. So the detection Depth should be Infinity, not 1 (and it should only detect calendars). I will consider this as an enhancement issue, thanks for reporting.