How could a bricked IMEI enter Apple's refurbishment system?

I recently bought a refurbished iPhone 8 directly from Apple. Apparently (after working through their tech support), the phone has been bricked by the previous carrier, and cannot be activated.

I'm curious about theories as to how this could happen. First, how could such a phone enter Apple's system? Presumably it was stolen, or never paid off, etc. Second, surely Apple is checking the IMEI numbers as part of their refurbishing process?

(The tech support call was a bit frustrating, since it was hard to convince Apple that there really was something wrong with the phone. Spectrum Mobile had gotten an error they were not familiar with when they tried to activate it.)