Signature image becomes featured image

Using the ‘Text for Message End’ tag (:end) ensures that the text which follows (for example in the signature) is not included in the post, however, if the signature includes an image, that becomes the featured image anyway. Is this expected?

The signature text I’m dealing with is too generic to target any parts of that using signature patterns; this would risk false positives in the message body.

Is there another way to target images in the signature to ensure that they do not end up being the featured image?