When attempting to set a Decal’s texture that is parented to any part in a Humanoid (e.g. head, torso, arms, etc) to an EditableImage, the texture does not render.
In some cases, adding a second Decal that has an Asset texture to the same face on the same part (even if it has Transparency set to 1) will cause the EditableImage Decal to render correctly, but this does not always work.
Removing the Humanoid or placing the Decal on a part outside of the rig that is welded to the rig is a workaround for this issue.
In the following screenshot, I’ve shown a normal rig using a non-EditableImage face texture, another rig with an EditableImage face texture, and a Part with a Decal that uses the same EditableImage. As you can see, the EditableImage face texture does not appear, but it does appear on a normal Part.
Thank you for the thorough bug report! Unfortunately, this current limitation is by design. Nonetheless, we understand how this is unintuitive and limiting, so we will convert this bug report to a feature request and add it to our backlog. Thank you again!