Hood, I understand what you mean.
But this is what I did:
The M30 would not load on it's own line, I clicked edit, highlighted the M30, hit space bar to delete it, typed M30 at end of last g0 line, clicked the red X to close, prompted to save, saved, back to mach screen and M30 is there. I never hit enter/return.
I'll try it a few times to be absolutely sure.
Hood, I did this several times.
As you say.. you must hit "enter/return" after the last line, but you can add to or change the info. in any existing line without having to hit "enter/return".