Updating:
I have been experimenting further and have managed to gain a connection with GRBBL-M3 device on Macbook Pro. Seems a bug if there is more than one device profile set-up in LightBurn.
A continued (intermittent) issue is that connection is not persistent; ie. I have needed to do a system and hardware reboot on a few occasions as I receive a ‘disconnected’ device status that cannot be resolved when changing to a new file. I cannot always replicate this issue / not consistent.
Full reboot and recreation of device profile seems to resolve but is annoying to have to do this.