Auto ISO was created as a way to keep your shutter speed at a certain minimum level in low light situations. It was never designed to be used with flash, and the introduction of flash in the equation is beyond the design parameters of Auto ISO.
Auto ISO is simply the wrong tool to use with flash, much like using a screwdriver to cut something in two instead of a saw is using the wrong tool. You will get results, but they won't be pretty.
Of course with film, Auto ISO was not event possible. So, set your camera's ISO to what film speed you used, and use your flash as you did before, and you should get comparable results.