Might be because the build system detects some issue and deletes the image because it would brick the device.
This happens commonly for devices with 4Mb of flash and the image does not fit anymore or devices where the kernel becomes too big for the kernel partition so the device would become unbootable even if it has large amounts of flash space.
You would need to check the fail logs (I don't know where they are for targets, you can try building locally and see what error you get) and open a bug report or ping a developer about it.
that's a very generic overview of things that will show failed only if the build for the whole target fails, not if some devices can't be built but most others are OK.