Background; there is a piece of software on your machine which takes a text string" G0 X27 Y18" and converts that to commands to move the motors. Let’s call that GRBL-basic.
GRBL-basic needs to know when that text string ends(End of Line-EOL) and another text string begins. So there are invisible codes put at the end of those text strings and are used for this purpose. Often they are either a carriage return char(CR), a linefeed character(LF) and sometimes both(CR/LF).
I am wondering if the EOL character used in the USB stream of text strings is different from the EOL character on the text file saved and put onto the uSD card. If you are using Windows Microsoft has done all kinds of things to try and guess at what you want to do. Like when you create a file called testing.conf and the file saved is really testing.conf.txt but they only show you “testing.conf” in the file browser(used to be called Explorer but who knows now). Software looking for a file called test.conf does not find one because there is only testing.conf.txt So I wonder if something changed the end of line(EOL) characters when Lightburn exported the G-Code to a file and now your GRBL-basic does not understand the G-Code.
That is why I was asking for the file, not a copy of the pasted GCode but the actual file. Copied off the uSD card would be best. I would even recommend you look at the number of bytes(size of the files) between the one saved to your computer by Lightburn and the size of the file on the uSD card.
GRBL-basic just works on those text strings and to get the strings to it there are 2 software modules, one reads from the USB connection and sends those to GRBL-basic and the other reads from a file on the uSD card and sends the text strings to GRBL-basic. So you see why this is a really strange problem…