From our Tag Assistant plug-in:
We have detected that the Google Tag Manager code snippet on this page is a non-standard implementation.
Using non-standard code implementation on a page may prevent tags from being fired.
Verify that your code snippet is working fine. You can make use of the preview and debug tool to ensure all tags added in Google Tag manager are being fired - https://support.google.com/tagmanager/answer/2695660?hl=en
Typically Google Validation Tools simply scrub the page and check to see if their code block is directly within the page source. If it is not, the tools will either say it is a "Non Standard Implementation" and/or it is not configured correctly, even though it works just fine.
Hope this helps.
As a side note the Tag Assistant tool changes constantly. There was a long period where Tag Assistant actually identified a Google tag as being deployed through Tealium, but they've since reverted to simply stating it as 'non-standard.'