If you call plot with an output argument to return handles to any lines that are created, you'll see that 100 individual lines get created, one per point.
x=1:100;
y=1;
h = plot(x,y,'DisplayName','a bug');
size(h)
That's also why your legend has 100 entries, one per line that plot created.
If you're using release R2018b or later and you want to display a horizontal line stretching the whole width of the axes, use yline.
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