excludeDecorators currently does not work as a per-<Source> prop, so your reproduction matches the implementation rather than an MDX mistake.

SourceProps does not declare, consume, or forward excludeDecorators. For the HTML renderer, source generation only checks:

context.parameters.docs.source.excludeDecorators

The working configuration is therefore a story parameter:

export const PrimaryWithoutDecorators = {
  ...Primary,
  parameters: {
    docs: {
      source: {
        excludeDecorators: true,
      },
    },
  },
};

Then reference both variants:

<Source of={ButtonStories.Primary} />
<Source of={ButtonStories.PrimaryWithoutDecorators} />

Because the value comes from the story context, it cannot currently vary between two Source blocks that reference the same story. For that case, use a second story export as above, or provide explicit code/transform for one block.

The HTML renderer reads the parameter here. The documentation describing excludeDecorators as a direct Source prop is therefore inconsistent with the current implementation, so a bug/docs report would be appropriate.