Good - you are on track again 
You already figured this out - unless your USB media is defective the USB may have been removed from the system writing the ISO to it, before the ISO was complete.
You can use our in-house get-iso script (in the repo as manjaro-get-iso) which will verify the downloaded ISO for you.
→ [INFO] New package - manjaro-get-iso application
If you are not aware of it - we have a package (udev-usb-sync) which makes sure that your system is kept occupied while writing to the USB device - meaning - it will not appear like the copy has been done - until it has actually finished.
→ [INFO] New package - udev-usb-sync
The latter has been tested for the past year and is included with 25.x ISO images.