A text with several stand-alone underscores is not converted correctly.

input

**_ _ _ N _ _ _ _ _ _ _ R S _ _ N _** 

output

 <p><strong><em> _ </em> N _ <em> _ </em> _ <em> _ R S _ </em> N _</strong></p> 

expected

<p><strong>_ _ _ N _ _ _ _ _ _ _ R S _ _ N _</strong></p> 

It seems that underscores with the surrounding spaces are replaced by the placeholder.
In the original on https://daringfireball.net/projects/markdown/dingus it is converted correctly.