Mike Bombich:

This week we reported to Apple a serious flaw in macOS that can lead to data loss when using an APFS-formatted disk image. Until Apple issues a macOS update that resolves this problem, we’re dropping support for APFS-formatted disk images.

What I describe below applies to APFS sparse disk images only — ordinary APFS volumes (e.g. your SSD startup disk) are not affected by this problem. While the underlying problem here is very serious, this is not likely to be a widespread problem, and will be most applicable to a small subset of backups. Disk images are not used for most backup task activity, they are generally only applicable when making backups to network volumes. If you make backups to network volumes, read on to learn more.

As Bombich points out, this is a fairly specific issue that may not affect many of you but it could be catastrophic to those of you it does affect. Thanks to Moeskido for the heads up.