The xmlSortAttributesByKey option is unstable when comparing an xmlns attribute with another prefixed attribute. For example, consider the following XML element:

<foo xmlns:foo="http://www.example.org" foo:foo="1" />

Formatting it with prettier --plugin=@prettier/plugin-xml --write --xml-sort-attributes-by-key swaps attributes:

<foo foo:foo="1" xmlns:foo="http://www.example.org" />

However, running the same command again, swaps the attributes once more, changing the code back to its original form.

The possible culprit of this behavior may be the following line:

There's a typo in the if statement—instead of xmlns, it's xlmns.