I'll add "Dietersheimer Weiher" as a synonym on Wikidata and leave it up to you, which name to chose for OSM :)
But if you have an official flyer with this name, that's probably more correct.
Thanks for noticing this btw!

I haven't added the wikidata name myself, but I'm certain that "am Hart" is the correct name.
So we should probably edit this? I might have overlooked this, when i added the Wikidata item.