Unable to override AT templates with Display Suite

I'm having trouble working with AT templates with Display Suite. I'm using DS 8.x-2.6, with DS Extras enabled.

I have a content type set up to use the Adaptivetheme Two Column: 4-8 layout. No matter what I do I can't seem to override the default AT template. Doesn't matter which of the template/filename suggestions I try, or where I put the template (in the AT directory, or my subtheme directory, under templates root, templates/layout or templates/content). Nothing works. FWIW, if I modify at-layout--twocol.html.twig directly I do see my changes.

Here's the Twig debug info from the source:

<!-- FILE NAME SUGGESTIONS:
   x at-layout--twocol.html.twig
   * twocol-4-8--node--###.html.twig
   * twocol-4-8--node-contenttype-full.html.twig
   * twocol-4-8--node-contenttype.html.twig
   * twocol-4-8--node-full.html.twig
   * twocol-4-8--node.html.twig
   * at-layout.html.twig
-->
<!-- BEGIN OUTPUT from 'themes/adaptivetheme/at_core/layout_plugin/templates/twocol/at-layout--twocol.html.twig' -->

Here's what I'm seeing on the Manage Display tab for the content type

Current layout--twocol

Adaptivetheme Two Column: 4-8
The default template can be found in themes/adaptivetheme/at_core/layout_plugin/templates/twocol

Template suggestions:
   
•twocol-4-8--node.html.twig
•twocol-4-8--contenttype-contenttype.html.twig
•twocol-4-8--node--{id}.html.twig

Note that the second template suggestion is not a type, I am seeing the contenttype doubled like that, which doesn't match any of the file name suggestions from the source. (I did still try that filename and it didn't work!)

Am I missing something, or doing something wrong? Thanks for any help you can provide!