Bleach: Thousand-Year Blood War shows the terrifying power of the members of the Sternritter, the powerful Quincies of Yhwach's Wandenreich army. Apart from their Seele Schneider and five-point cross, what makes them more frightening is their Medallion, which can steal a shinigami’s Bankai.
Bleach: Thousand-Year Blood War Episode 4, titled Kill the Shadow, showed various Captains of the Gotei 13 battle members of the Sternritter while Ichigo Kurosaki is also fighting a Sternritter in Hueco Mundo. While several Captains had their Bankais stolen during their fights, Quilge Opie, the Sternritter fighting Ichigo, is surprisingly not able to steal his Bankai.
In the first episode of Bleach: TYBW, Ebern also tried to steal Ichigo's Bankai but failed, and the villainous Quincy didn't know why. It looks like Ichigo is the only one whose Bankai couldn't be stolen by the members of the Wandenreich, but why?
Keep on reading to find out the answer, but note that there are major spoilers from Tite Kubo's Bleach manga series below.
Why can't the Sternritter steal Ichigo's Bankai?
Ichigo is a unique individual in the world of Bleach. He is a being who possesses distinctive characteristics from the four major spiritual races known to Soul Society (Human, Shinigami, Hollow, and Quincy). He has become a divergent being, making his existence harder to comprehend through objects such as the Quincy medallion.
The structure of Ichigo's Bankai is clearly different from any of the Captains'. Ichigo's weapon is simply a spirit sword manifested by Zangetsu, the embodiment of his Quincy powers acting as a shinigami's Zanpakuto. So pretty much, Ichigo's Shikai and Bankai are Quincy abilities functioning as shinigami powers.
As the Quincy medallion wasn't designed to steal Quincy's powers, it was impossible for Ebern and Quilge to steal Ichigo's Bankai.
In the manga, Yamamoto, the Captain-Commander of the whole Gotei 13, hypothesized that Ichigo's Bankai couldn't be stolen because it still has room to grow and evolve, but he was utterly shocked when Yhwach told him that in order to steal a Bankai, one needs to be sufficiently strong enough to steal it and shows this by stealing his own Bankai from him.