Turns out iX took a clearly-marked, non-mainline "work in progress" patch to enable asynchronous copy-on-write and applied it to OpenZFS... then shipped it as stable to everyone in TrueNAS 12. And it's been silently corrupting data. Check your files, TrueNAS users - and upgrade to the bug-fix release, or disable async CoW manually! Bug tracking ticket.
yeh, this one is seriously WTF. Though Truenas brakes other things also, so wont be updating from 11.x anytime soon.
Adds one more maintenance item to my system. Top of the list was migrating Plex from the jail created when the system was new (9.x I think) but this now tops the list.