Based on questions I've asked previously, I always tend to rate my films at less than their box speeds - typically 50% less, so if it's ISO100 I'll rate at EI50, or thereabouts. With this 125 film, I rated it at EI80 to try and ensure I got a denser negative than previously. My SB-800 was set at -1 power using TTL mode (not TTL-BL as I was only interested in the face or main subject) and the flash mode was SLOW+REAR so it just acted as fill in flash.

Seems to me you rated it at MORE than box speed. If you wanted denser negatives you should have rated it at 64. Also, flash used on camera fills in the shadows and makes for a flatter negative.