I have no idea how the ACL option became disabled. I'll turn that back on.
As for the "fix", when several TB of data is affected, copying to other drives isn't exactly an option...
Now I've gone and enabled the ACL and I have zero access to this directory now. Can't take ownership or even see the security permissions. Excellent.
I suppose I need to sink a few hours into picking apart the array again, manually resetting permissions and rebuilding the pool.