I have a sector leader with mining 20 percent bonus but on the mining screen it says 1 for sector (and 1.05 for governor which is correct).
Is the system definitely assigned to the sector?
Yes, BUT MAYBE NOT WHEN I POSTED THAT SORRY. First of all when opening the game just now to show you I got this;

then when I checked again it shows the sector bonus properly now as 1.2. Weird as I am sure it was displayed wrong earlier but maybe I didn't put the planet in the sector yet.
I keep trying to attach the DB on here for the above error message but it's not posting for some reason. Let me know if you want the DB.