I FOUND THE SOLUTION to using Apple Disk Utility to create RELIABLE disk images of APFS drives. It was not easy to figure out. The ONLY path to make a viable re-usable disk image formatted as APFS is:
1. First create a blank image file formatted as APFS and Read/Write (requires pre-determining size of image file).
IMPORTANT NOTE: The image (destination) cannot be stored on the same drive or partition (source) from which you are going to create the image.
2. use Restore from <volume> to clone to newly created image file
(Highlight destination <volume> then click [Restore] button and select Restore From <volume>.
3. convert image file to Compressed format (Image > Convert).
ONLY THEN do you have a reliable APFS image, especially w/bootable macOS. I have done this repeatedly and it works fine.
ANY other options/paths, are greyed out, yield "Resource busy" or "image is not APFS format" errors or produces image that is not reliable. Even using Apple Internet Recovery tools/boot was the same.
NOTE: you do NOT need to do ANY of this using Recovery tools. You can do this from a live boot of macOS drive. If you do not have a destination to store the image file other than the same internal drive, you can create a partition large enough to store the image file, using Disk Utility. And yes, you can partition macOS boot drive live.
CAVEAT: Tested only using Mojave 10.14.4 on 2019 27" i9 iMac w 40GB RAM and 512GB SSD (no T2 chip), so YMMV.
Apple needs to fix Disk Utility to work straightforward like it used to but with better support for APFS.
HTH.