Hi David,
There are two ways to flash application image by MCUBootUtility.
#1 Set image file (srec/hex/bin/axf/elf) path in "Application Image File" input box and click "All-In-One Action" button

#2 Set image file (only bin file) path in "Bin File" input box and click "Write (Auto Erase)" button

There are below differences between these two ways:
1). #2 can only flash bootable image (with FDCB, IVT, BootData header), and the format must be binary (.bin). #1 can flash both raw image (only app, no header) and bootable image, and the format is unlimited.
2). If provided image file is bootable image, #2 just flash this bootable image without any changes. #1 will only keep app and DCD, for FDCB, IVT, BootData, Tool will try to re-generate them according to settings in 'Boot Device Configuration' and image self
so for your case, you can use #2 to flash image or set 'Boot Device Configuration' properly before using #1
Best Regards,
Jay