AnyCubic Chiron shifting in Y axis
AnyCubic Support suggested I swap two of the motor drivers (X and Y) and after I did that, the problem disappeared.  I've had a half dozen good prints in a row now, no problems at all.

The drivers are inside the computer module - had to flip over the printer, take off the cover and pry out the top two modules and put them back in swapped.  Not difficult, but probably worth getting advice from AnyCubic before going ahead and doing it yourself unless you're comfortable with this sort of thing.