Can somebody provide a canon answer regarding if/when Voldemort knew Snape had the ability to create a Patronus? Seems like "He Who Must Not Be Named" would stop trusting anybody that could make one as soon as he found out. Inspired by and linked to this question.
Dumbledore stated that Voldemort never bothered to learn about things he viewed as worthless. Love was one of those things. So, I wouldn't think that Voldemort would bother to care about the origins of Snape's Patronus.
Second, Patronuses were not necessarily the exclusive domain of the good guys. Dolores Umbridge had one, a cat, protecting her from the Dementors in the Ministry dungeons. While not a Death Eater, there was never any love lost between her and the Order, HH&R included.
Third, Voldemort did know that Snape had feelings for Lily. He said as much when he and Harry were staring each other down at the end of Book 7. Again, Voldemort considered it beneath his notice; he didn't understand love, so he disregarded it.
Lastly, Snape had wanted the DADA job since Voldemort disappeared. Being a good DADA teacher and thus fit for the post would require being competent with defensive spells, including Patronuses. It would have been trivial to explain to Voldemort that Snape had a corporeal Patronus simply because he needed one in order to have a chance at the DADA post.
Harry suspects he never saw it.
Though it’s never addressed if the Dark Lord knew that Snape could cast a Patronus, the closest mention is when Harry asks the Dark Lord if he’d ever seen Snape cast a Patronus, and the Dark Lord doesn’t answer.
“You never saw Snape cast a Patronus, did you, Riddle?’
Voldemort did not answer. They continued to circle each other like wolves about to tear each other apart.
‘Snape’s Patronus was a doe,’ said Harry, ‘the same as my mother’s, because he loved her for nearly all of his life, from the time when they were children.”
- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)
From this exchange, it seems like Harry presumed that if the Dark Lord had seen Snape’s Patronus, he’d have realized that Snape was in love with Lily Evans and suspect he wasn’t truly loyal. However, there’s nothing that truly proves if the Dark Lord knew Snape could cast a Patronus, or if he’d ever seen it - Harry wasn’t necessarily right.