Expansion only works on a healthy pool. So here expansion was suspended until the 12-wide raidz2 had fully resilvered, and then it took over where it was. And expansion does rewrite existing data across, even though it does not refow existing data to the new stripe width, so it is bound to take quite some time for a pool which holds over 80 TB.
As for space accounting after expansion, that’s indeed deeply borked.
12-wide raidz2 is already at the upper boundary of what’s considered safe, so you’re pushing it. Any further resilver and/or expansion is going to take forever and some days.