Maybe a little more detail how you installed? Login as user "installer" and things? :-)
I had similar issues with a Lenovo, my fix is at the end of this thread:https://forum.opnsense.org/index.php?topic=32485.0
What would be interesting is to see post install of OPN what the partition tables are for each drive. Sounds like the installer despite being told to use both as a zfs mirror, might be only applying it to one and the boot sequence looking at the other.Of course if this is unmodified freebsd code that would not happen, as is dealt with by the underlying zfs code, IMHO it would be interesting none the less.Or, there is missing bootcode for some reason in the partitions.
Unfortunately it's a bit too vague to do anything here.