This is a completely understandable oversight by the characters. Im sure Din doesn't think he fought the cyborg, he was captured and disarmed. And the cyborg never really took possession, just tossed it away. Plus even if it technically counts its, the story that has power, and thats not a very good story.
If the saber is stolen and you defeat the thief, does that count? What if you hire the thief so you don't have to fight the current owner? Its the story that matters.
Technically... Luke or Ahsoka should have the Darksaber, I think, if it just goes by the defeat of the owner.
Pre Vizla had the saber, but was killed by Maul, so Maul was the rightful user. Maul left it at his former base & it was FOUND by Sabine Wren & she took it, was trained by Kaanan, & then GAVE it to Bo Katan. That's why most consider her reign cursed, because she didn't earn the right to wield it. Then, of course, Moff Gideon beat her & took it until he was defeated by Din.
Now, if it's like the Elder Wand, & has to be won by defeating the owner, then Darth Maul would've still been the legitimate wielder until he was defeated by Ahsoka, but since that wasn't strictly one-on-one combat, I'm not sure it counted, but if it does, then she's the owner.
Let's say that DOESN'T count. Then Maul would've been legitimate until he was killed by Kenobi in straight combat. It would've then passed to Vader who beat Kenobi, then to Luke, who defeated Vader.
Only caveat with all that is, it's not been made clear if the Darksaber has to be used in the duel that leads to the defeat. If that's the case, then IDK, because Tar Vizla, the first Mandalorian Jedi, never lost it. It was taken to the Temple by the Jedi after his death & later stolen by the Mandalorians.
Come to think of it. THERE'S a flashback I'd love to see... A fully united force of Mandalorians assaulting the Jedi Temple during the prime of the Jedi to take back the Darksaber.
Shoot... Better than that... Give us either a movie or a one season show, ROUGE ONE-style, about the planning & execution of the assault.