If you are using a version of GRBL firmware older than 1.1f, it doesn’t support variable power mode.
Read here: https://github.com/gnea/grbl/wiki/Grbl-v1.1-Laser-Mode#laser-mode-operation
LightBurn’s “GRBL” profile uses the M4 (dynamic power) mode, so that the power is varied with the speed of the laser head. The “GRBL-M3” profile uses M3 (constant power) mode, so the power output is constant as the speed of the laser varies, resulting in darker burns at start & stop points, as well as any corners.
It would be possible to extend the start and stop points, but it would be hard to add them to all corners in any reasonable manner.