Rumor: it's in Ferdinand's German wiki entry, I think, but from what few mentions I could see in books, it doesn't seem to be based on more than widely shared dislike of his wife, love of gossip and the fact that Ferdinand post 7 Years War never lost the habit of appearing sickly in public. Lehndorff - who doesn't like Mrs. Ferdinand, repeatedly laments in volume 1 that marriage has changed Ferdinand, and much later suspects some of her kids being attentive to Heinrich because they're after the heritage - in the 1799 journal makes zero mention of this, and not in the previous journals, either. I'm assuming if he thought there was something to it he'd have mentioned it. Otoh, despite his dislike of Mrs. Ferdinand, he does mention she fusses over her husband and making sure he's always warm and comfortable when Lehndorff visits in the winter of early 1799, which is decades after they married, and Lehndorff isn't someone she'd want to impress at this point.

Karl Emils are doomed, clearly. At least when born into the Hohenzollern clan.

Well, Fritz as a star (making the others planets or comets?) is a far more flattering imagery than Fritz as an iron chain shackling involuntary laborers!