Re-locking is often because it may brick the device if any custom partitions remain.
If you try to run fastboot oem lock on a device where the 1662 exploit was used to unlock, you’ll often get: se bootloader unlocking relocking 1662 patched
Before the patch was widespread, unlocking the SM-G925F was a straightforward process via Samsung's official tools. If you have an older, un-patched device, you can still use this method. Re-locking is often because it may brick the
When users refer to a device being "1662 patched," they are usually referring to a security update that patched previous bootloader exploits (such as the "l exploit" or "loo exploit" used on devices with DRM issues). When users refer to a device being "1662
For non-Samsung devices: “1662” could be a (e.g., 2016-06-02) or a build number suffix where a bootloader unlock exploit existed.