You can wonder...
Many people here will tell you, you had just to read the warning after the zpool upgrade
command.
That's always made me "laugh" as it's somewhat cryptic now and it was totally confusing before, speaking of gpart bootcoode
even if you didn't use BIOS booting at all, but efi.
Worse, it was giving an example that's not suited to your disk configuration. How many people didn't boot because the partition index wasn't correct for them?
How many people clean their disk from FreeBSD because of that?
In true, I don't laugh because FreeBSD is a good OS and it doesn't deserve that problem which is just bad communication and... Well, lack of efforts to make that understandable.