>> "keyboard commands to "STOP" seem to be mere suggestions to stop."
That seems to be a good explanation.
I'm no expert in the SB internals but from my experience, the stop command (space bar) is simply dropped into the top of the queue for execution. So if the executing command is jogging 50 inches, the only way to get it to stop moving is to hit the big red button. Once the command is complete, the space bar stop will be executed. Potentially, a little late.
The good news is that if the job is rapidly executing code lines, the space bar stop is quite effective.
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2013 PRS 96x60x12 (Centroid upgrade)
4hp Spindle
12" indexer
Aspire
Rhino
Fusion 360
Ferrari 360
Prusa MK3S+
Prusa XL multi-tool