(xml) line with single letter namespace prefix not highlighted since 10.4.0

[boghyon]

Are you using highlight or highlightAuto?
highlight

Sample Code to Reproduce

<abc:OK xmlns:abc="..." />
<ab:OK xmlns:ab="..." />
<a:FAIL xmlns:a="..." /> <!-- ← No syntax highlighting for you -->
<OK xmlns="..." />

https://jsbin.com/gexeyen/edit?html,js,output

Expected behavior

<abc:OK xmlns:abc="..." />
<ab:OK xmlns:ab="..." />
<a:OK xmlns:a="..." />
<OK xmlns="..." />

<abc:OK xmlns:abc="..." />
<ab:OK xmlns:ab="..." />
<a:OK xmlns:a="...">
  <AllOK xmlns="..." />
</a:Ok>
<OK xmlns="..." />

Additional context

• Reproducible since version 10.4.0.
• Initially reported on Stack Exchange (now moved to this issue).
• Same behavior on Stack Overflow: https://stackoverflow.com/a/48559689/5846045 (See <f:FlexibleColumnLayout>).