@MikeH Nope. No clue. Sorry.
I ran the bad one in air using gSender 1.2.2 and Edge 1.3.8. Both stopped after running for 400+ lines. I could click on resume and it would run for 3 lines, then stop. Etc, etc. The only thing different that I could see on those lines were I and J commands, which are arc commands. I looked at a few of my files and they did not have those commands. I thought that I had a Eureka moment. Wrong! Your file that runs fine also has I and J commands in it.
So, in short, I struck out. There are members on here much more knowledgable about grbl than I am. With luck, they will chime in with the definitive answer.