I also think it is a deformed Persicaria lapathifolia achene. It has a similar colour and surface texture as the species, and you found other deformed achenes in the sample. I wonder if the deformed seed has the 2 stigmas at the top? That might be an additional confirmation.
I think your image of the collected achenes is excellent, and would be a good example for other seed analysts. Do you have a larger version available?
yes, the deformed seed has stigmas at the top, thank you. I am in the process of expanding my photographic collection of atypical seeds or without testa or with their fruit. in this moment I don’t have many shots but I will be happy to share this information with other analysts. Do you want to share it on this site, maybe in the forum “seed anatomy” or in others ways?