Going to @Aaron.F as well on this.
It’s the line length. I tested with my Snapmaker and XTool and it stops on both. I tested the symbols and changing out a few things before I figured if I trimmed it short, it ran fine. It seems to have some form of 127 byte/character limit on lines.
;0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123
Adding this comment, which is 125 characters/bytes (; + numbers) runs fine. Keep in mind, this is 127 characters with two hidden for the return.
However, just adding another digit, like 4 to the end.
;01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234
This will cause it to hang up. 126 character line, which then also would contain a non-printed carriage return and line feed (which you can see by toggling all characters in a rich text editor such as Notepad++, see below) makes it 128 bytes.
So the limitation is some form of limit on how long the line can be when running a project via USB. I don’t know if this is a limit in Lightburn or serial transfer method.
EDIT: Or if it’s a limitation of the command buffer size. Maybe make an option in Lightburn to ignore thumbnail lines?