APFS Fusion Drive with external USB on 2018 Mac Mini
This is a bad idea for lots of reasons. You're lucky that the Mac can boot from a Fusion drive that's half T2 and half external USB.

I suspect the T2 disk controller is at the root of the problem. Possibly because of its built-in encryption. Perhaps the Fusion magic moves encrypted blocks off the SSD without decrypting them?

Even if it did work, using a USB thumb drive is going to massively compromise the speed of the  fast internal portion. 

I don't think that 'expanding' the internal storage into a bigger logical volume, either with Fusion or RAID is going to be workable.