\not mis-places the line

[retsyo]

\not \in \qquad,\qquad \not \ni \qquad,\qquad \not a \qquad,\qquad \not\equiv \qquad,\qquad \not\operatorname{R}

should displays as

but now in DOCX, it is shown as in other words, the / is not over-drawn on the latter character

[retsyo]

no, the generated mathml string does not place / on characters no matter in https://www.tutorialspoint.com/online_mathml_editor.php or DOCX

[roniemartinez]

Did you update your version?

The output for \not \in is ∉ which is the unicode for ∉ which is equivalent to \nin.

[retsyo]

yes, I am using latest cloned version

however, the produced mathml is rendered corrctly on https://saxarona.github.io/mathjax-viewer/

[roniemartinez]

The way to verify that you are running the correct version is when \not \in and \nin have the same output.

$ l2m -b -t '\not \in'                                                                                                     
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><mrow><mi>&#x02209;</mi></mrow></math>

$  l2m -b -t '\nin'    
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><mrow><mo>&#x02209;</mo></mrow></math>

You can use the MathJax viewer from their official website: https://www.mathjax.org/#demo