yallala4535453 If fastboot is saying the bootloader is locked, then I don't see how any app can detect that it's unlocked. If an app is saying your bootloader is unlocked anyway, then it's possible it's just a generic message that's popping up after they detect the OS isn't certified by Google.
You can double check that bootloader unlocking is disabled in developer options as directed here in the instructions, but I don't know if apps can see whether that's enabled or not.
As for whether you can modify those props without breaking anything, I don't personally know. I would imagine that you can't change them. Regardless, I wouldn't suggest modifying or attempting to modify those values if you don't know what will happen.
Finally, I am sure GrapheneOS developers can hide the fact that the bootloader is unlocked from apps, but I doubt they'd do that. Locking the bootloader is the last step of installing GrapheneOS. Keeping the bootloader unlocked isn't secure. If random apps are throwing up warnings, then they're really helping remind those who haven't locked their bootloaders.