sampler2D triggers schema validation errors in Collada 1.5

Using instance_image on sampler2D triggers validation errors. Something feels off with the schema.
Keeping sampler2D empty validates the schema.

Error:

xmllint --noout --schema http://www.khronos.org/files/collada_schema_1_5 issue.dae
element sampler2D: Schemas validity error : Element '{http://www.collada.org/2008/03/COLLADASchema}sampler2D': Character content is not allowed, because the content type is empty.

Test Case:

<?xml version="1.0" encoding="utf-8"?>
<COLLADA xmlns="http://www.collada.org/2008/03/COLLADASchema" version="1.5.0">
  <asset>
    <contributor>
      <author>issue</author>
      <authoring_tool>issue</authoring_tool>
    </contributor>
    <created>2021-03-24T13:32:13.351Z</created>
    <modified>2021-03-24T13:32:13.351Z</modified>
    <up_axis>Y_UP</up_axis>
  </asset>
  <library_images>
    <image id="sc-3-d-shelly-v-2-01-ktx">
      <init_from>
        <ref>sc3d/shelly_v2_01.ktx</ref>
      </init_from>
    </image>
  </library_images>
  <library_effects>
    <effect id="character_mat-effect">
      <profile_COMMON>
        <newparam sid="sc-3-d-shelly-v-2-01-ktx-sampler">
          <sampler2D>
            <instance_image url="#sc-3-d-shelly-v-2-01-ktx"/>
          </sampler2D>
        </newparam>
        <technique sid="common">
          <phong>
            <emission>
              <color>0 0 0 1</color>
            </emission>
            <ambient>
              <color>0 0 0 1</color>
            </ambient>
            <diffuse>
              <texture texture="sc-3-d-shelly-v-2-01-ktx-sampler" texcoord="UVMap"/>
            </diffuse>
            <index_of_refraction>
              <float>4</float>
            </index_of_refraction>
          </phong>
        </technique>
      </profile_COMMON>
    </effect>
  </library_effects>
  <library_materials>
    <material id="character_mat">
      <instance_effect url="#character_mat-effect"/>
    </material>
  </library_materials>
  <library_visual_scenes>
    <visual_scene id="default-scene">
      <node id="CHARACTER" sid="CHARACTER" type="NODE"></node>
    </visual_scene>
  </library_visual_scenes>
  <scene>
    <instance_visual_scene url="#default-scene"/>
  </scene>
</COLLADA>

People mentioning this problem:

1. https://community.khronos.org/t/load-textures-in-collada-1-5/104653
2. https://community.khronos.org/t/schema-problem-with-sampler2d/6329

Start by validating the supplied XML test case with xmllint against the Collada 1.5 schema and compare the sampler2D definition with the two linked Khronos discussions. The work is done when the schema's handling of instance_image inside sampler2D is corrected or the expected schema behavior is documented and verified against the test case.