Drobo actually put in a method to create a Time Machine partition, but to macOS it looks like an HFS+ partition.
Have this uncomfortable feeling they're still nowhere near offering APFS support, it appears that they have to write some kind of translation system to make it compatible under their proprietary BeyondRAID protocol, and it looks like whoever they hired to do HFS+ for them doesn't want to do it again for APFS, or Apple isn't giving them enough support (which seems unlikely since the SoftRAID guys are supporting APFS) or some other political reason.