This is discussed on page 28 of Montgomery's 2009 Spell of the Tiger: The Man-Eaters of Sundarban. (Having said that, who knows? Montgomery is writing about things people told her and things she saw, i.e., seeming abandonment of the masks. But this is a start ...) (In Blood Rites, Barbara Ehrenreich takes Montgomery's account as fact but simply uses the point in passing.)
Here's the passage from Montgomery:
The masks worked as long as the tigers believed in them. Which was not for long. “After five or six months, they were finding out that this was not the front of the human being,” Kalyan Chakrabarti told me when he visited us at the Tollygunge Club.
Kalyan was an intense and fervent man in his fifties, possessed of a dense energy, as if all his intuitions and theories and stories and plans were physically compacted into his short, stout body. He was adamant on this point: “They know what a human being looks like,” he insisted. “They know there is a back and a front. Then they are finding out that one is not a good front.” The masks, he said, were “a little gimmick that worked for a particular period.”
(When we visited, although some officials still considered the masks effective, almost no one seemed to use them. Of all the villagers I met in Sundarbans, I found only one fishing group carrying the masks on their boat.)