Ahsoka Tano, padawan to Anakin Skywalker aka Darth Vader is a formidable opponent for Thrawn. Thrawn outmaneuvered her as he knew Anakin and his strategies. Without the information of her master, even a grand admiral as shrewd as Thrawn could not outsmart Ahsoka. Thrawn was right. He did not negotiate with the apprentice of Anakin Skywalker. He knew Anakin as a Jedi, and he knew him as Darth Vader (Yes, he knew, it was confirmed in the novel

Thrawn: Alliances). In both his persona, Thrawn knew him to be quite dangerous and unpredictable. That is why he was so wary of his apprentice.

After his exile to Peridea, Thrawn could never underestimate the force wielders again. A single Jedi like Ezra Bridger outflanked him. Thrawn knew he could not be defeated easily. He normally calculates everything if he knows their pattern. He cannot do it when the pattern is broken. Force is an arcane thing for him. But his approbation for Vader remains…