Is it possible that this is the result of the driver ignoring the attention "nags"? Seems more plausible than FSD having a brain fart (unless many other FSD Teslas are making that same decision in that same spot). But even so, is Tesla's fail-mode really that unsophisticated? Pulling off into the left lane, even if it's the nearest off-lane, seems problematic, especially if it's in front of another vehicle who is at full speed b/c it had (until that moment) a clear road.
I don't expect FSD to be able to find its way to a proper rest stop or off-ramp, but seems like a less dramatic shutdown mode should be possible, e.g. driving a couple of miles at moderate speed until there's a safer pull-off opportunity.