Upgrading from Android version 6 to version 7
Bootloader is unlocked
Phone is rooted
After upgrading from v6 to v7 the OEM Unlock is greyed out as if shut off and locked. This is apparently a security feature from the upgrade.
Note – you do not need to do anything. Your phone is still unlocked, it just looks like it is not.
If you want to regain control of the slider, (which is not really needed), you need to relock your boot loader and then unlock it again. This will obviously wipe your phone.