By RFC 6352 6.2.2 “CARDDAV:supported-address-data Property”, CardDAV servers MUST accept VCard 3.0 and only MAY accept 4.0. However, DAVdroid always sends VCard 4.
Strictly speaking this is a bug, but I will consider it as an enhancement to detect whether the server is VCard 4-capable and then choose the right format because most servers/clients understand VCard 4 today.