Under the TACTICAL tab of the CUSTOM DIFFICULTY settings there is an option called PLAYER TOUGHNESS. In the latest patch this has been linked with Character Skills and Weapon Upgrades. If the option is set to either Authentic or Authentic Plus then the Weapon Upgrades will be disabled, which will include your Mosin Nagant Zoom upgrade.
Changing the PLAYER TOUGHNESS option to a difficulty lower than Authentic will bring back the Weapon Upgrades, but only after a level restart.
We are looking at ways to improve this in a potential future update