Hey I,ve solved my problem! Don't know the reason , but when I connected SSD to second slot available on main board, Mac mini restarts and no question mark is seen anymore.

Hey, I have same problem. Hardware mac mini late 2012, Intel I5 2.5Ghz, 16GB RAM 1600Mhz, SSD Samsung EVO 870 6Gb/s. MacOS Catalina. I did everything: 1) cleaned disk and made clean install 2) wiped NVRAM 3) Run partition repair for both partitions - not found any errors at all and nothing help. System while reboot it just freezes up and display question mark. That not happen when I turn MAC off instead, then starts up as a charm, quick as !&&*. Problems appears only with restarting, its like computer doesn't see any disk during reboot. During startup i pressed command until menu pops up, selected main SSD, so it start to load but when progress bar is close to 80% it ends up with same question mark on the screen. What helped was a replace SSD with Kingstone A400 6Gb/s - much slower disc than Samsung, but it works just fine, rebooting works well nothing unusual but noticeable slower. Do you think upgrading with Samsung is not for Late2012? Can u help somehow?