Criteria for thumbnail building is largely deferred to Discourse core now.
Your issue is likely to occur with any add-in component that uses Discourse thumbnails. Should they broaden their criteria?
Could you post the complete raw contents of a problem post? If I ever get time (I can’t promise anything) I’ll see if it’s trivial to broaden inclusion. However, I’m a little reluctant to override this as Discourse having implemented this in core is a good reason to avoid overrides and rely more on core. This will help stability of the plugin and reduce breaking changes. It’s exactly this kind of customisation that trips up plugins.
You could always maintain your own fork and customise this yourself?
You could try to use the thumbnail picker in the Topic Meta editor to see if you can pick the thumbnail, but that is based on the same criteria for inclusion. If the thumbnail doesn’t appear there, it is not meeting the criteria, so that would act as a good confirmation.