Cloning macs within a context of no internet access
I'm assuming you've disabled SecureBoot, and that the volume name is identical in what follows:

It's possible that the T2 chip on these new (2018) Minis makes 'straight' cloning difficult. 
I had the same problem when I cloned my old 2012 Mini's volume onto my 2018 Mini : I too got a flashing question mark. I was able to fix it by installing Mojave on top of the newly cloned disk. I used a USB install media (after disabling SecureBoot!). The disk booted perfectly after that, with all my files still in place.

I tried blessing the startup volume (though I may not have used the correct commands); and resetting NVRAM, without luck. I can only assume that the T2 chip 'knows' something has been changed and it requires an Apple installer to reset. 

It may be possible to perform the 'reset' without having to do an entire OS install, but I don't know what the procedure might be.