That sounds interesting. Maybe it is indeed related to LB is double sending commands (now with video)
@JohnJohn you might also take a look here. It’s both with grblHAL as firmware. Is there a change in the serial protocol between 1.4 and the following versions?
Personally, right now, I have many problems connecting to a recently compiled grblHAL-firmware with LB, but LaserGRBL works without issues.