Not a Nucleus user myself, but sorry to report, it's a known issue, (with Logic only). Seems to stem from the way Logic deals with MCU &/or Dante. It was supposedly fixed in Logic 10.2.1, so might be worth upgrading if you haven't already, (although some users are still experiencing the issue in later versions of Logic).
https://support.apple.com/en-gb/HT207473
"Level metering shown on a connected SSL Nucleus control surface now matches metering displayed in the Logic Mixer".
Probably worth contacting SSL support (if you haven't already)...