I would say that dropping support for APFS-formatted images, in general, is a good idea until the thing can be reviewed and tested properly. Who knows what other cans of worms are lurking under the surface.
It might be an isolated bug, but given this is backup software, I'd play on the safe side as well and not use APFS images until they have been fixed and thoroughly tested. Anything that is mission critical, like backups, should not rely on known buggy software. If it isn't 100% reliable, neither is your backup!
That is why you use tried and truted technologies for backups and you test them, you test them again and you test them regularly, once they have been implemented. You don't want to wait until you have a disaster, only to find that your backups are actually zero bytes long or corrupted!