I see a problem there: you have put 2 different OSes on the same HD. You should know that the boot part of your HD is completely related to the OS you wanna boot. So IMHO you should keep things simple: 1 HD = 1 OS. It's what i do personally, and i have no problems booting Linux, Win10 and Mojave on the same and only mac mini 2018.

EDIT: I'm not meaning that OS is the only thing you can put on your disk, you can obviously add several data partitions on the same HD... But the OS, always on the first partition, have to be the only one booting (active) partition of the HD.

 

I dunno. Never had issues with running more than one OS system on the mac's internal hdd. On this late 2012 mini I use. I have Sierra for my DAW and other main apps I use and I had a partition with Mavericks to use photoshop and lightroom licensed. I understand people will put a test partition on the internal (as I'm doing now).
I only dleted the Mavericks one at the moment to put the High Sierra test on there because apparently the Superduper clone wouldn't boot for me. However, that issue has seemed to be solved using CCC

 

Last edited: Oct 18, 2019

Yeah, I've never run into that problem either. I have one 8tb drive which has a (third redundant) backup of 2 of my laptop systems (one running mt lion, one el cap), and two different 5,1 towers running mavericks, and i have booted from it to check afaik as i always do...

maybe something with later osx's , but hasn't been a problem for me..