I had the same issue after my update to the new Version 6.12.6 (from 6.12.4). I found these fix and replaced the Files with one of the download Page, but I had the same issue. (used the 6.12.6). After this I downloaded the 6.12.5 an replaced it with these Files. Also the same.
At the end I downloaded the 6.12.4. After replacing the bz Files with this Version I could boot from my Stick.
Could someone explaine this Problem? I want to update in the hope that I get a better ZFS performance for my Cache. (Mover read with 70mb (35mb each SSD) from Cache Pool)