Events from pulldown-cmark:

"__!_!__" -> [
  Start(Paragraph)
    Start(Strong)
      Text(Boxed("!_!"))
    End(Strong)
  End(Paragraph)
]

Events from commonmark.js:

"__!_!__" -> [
  Start(Paragraph)
    Text(Boxed("__!_!__"))
  End(Paragraph)
]

XML from commonmark.js:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <text>__</text>
    <text>!</text>
    <text>_</text>
    <text>!</text>
    <text>__</text>
  </paragraph>
</document>

I think the problem here is that __ in "__! does not count as a left-flanking delimiter-run because neither 2a nor 2b are fulfilled:

• (2a) not followed by a Unicode punctuation character, or
• (2b) followed by a Unicode punctuation character and preceded by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.

Because __ is not a left-flanking delimiter-run, it should not be parsed as strong emphasis.