Saxon exclude-result-prefixes dropping namespace for attribute values

I have this weird problem with Saxon's XSLT processor (latest version = 9.?)

Given this bogus XML (doesn't matter what it is):

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="urn:bogus">bogus</root>

With this XSLT:

<?xml version="1.0" encoding="UTF-8"?>
<x:transform version="2.0"
  xmlns:x="http://ift.tt/tCZ8VR" 
  xmlns:b="urn:bogus"
  xmlns:xs="http://ift.tt/tphNwY"
  xmlns:xsi="http://ift.tt/ra1lAU"
  xmlns="urn:foobar"
  xpath-default-namespace="urn:foobar"
  exclude-result-prefixes="#all">

 <x:output method="xml" indent="yes" encoding="UTF-8" />   

  <x:template match="b:root">
    <foo xsi:type="xs:string"><x:value-of select="."/></foo>
  </x:template>

</x:transform>

Will actually drop the xmlns:xs="http://ift.tt/tphNwY" declaration from the resulting XML because of exclude-result-prefixes="#all". Apparently this happens because it is used in an attribute value and not in an element or attribute.

Removing exclude-result-prefixes="#all" or specifying manually which prefixes to remove does work.

Is this intended behavior? I guess the processor treats all attribute values as random strings and doesn't take namespaces into account for them (it doesn't even notice it has one I assume).

Should there be special behavior for well known attributes like xsi:type?