Wouldn't you know it, there was a little bug in the PTINV command, introduced at the last minute. Figures.
When using an existing surface, it erroneously used the Datum Adjustment value from the other routines.
I uploaded a patched version - it's the "SincPac v1.00a" version. Evidently, I can't delete files from the Lilly Pad...? So the original is still there, too.