The Mishna (Demay 7:3) gives the case of a worker who doesn't trust his employer to separate tithes. The worker has to separate tithes himself, i.e. 1% (1 fig out of 100) for terumat ma'aser, and ma'aser sheni which he then redeems on a coin. There are three opinions (as explained by the Rash, followed by Bartenura):
Tanna kamma: The worker has to set aside a fig uneaten to make up for the loss of one fig he causes to the owner by taking the tithes from the owner's property.
Rabban Shim'on ben Gamli'el: He has to set aside a fig, but has to buy another one, because not eating one would harm his ability to work for the employer.
Rabbi Yosi: He doesn't have to set aside a fig because of the stipulation of the court (תנאי בית דין), which requires the employer to pay for terumat ma'aser.
If the court made a stipulation, I would assume that it would be public and not something known only to Rabbi Yosi. The first tanna of our mishna and Rabban Shim'on ben Gamli'el, as well as Rabban Gamli'el, who fed his workers demay (Demay 3:1), all seem to have been unaware of it.
Rabban Shim'on ben Gamli'el, who was the prince, had been in controversy with the head of the court, Rabbi Natan (Horayot 13b).
Is it possible that Rabban Shim'on ben Gamli'el didn't accept the court's stipulation? Alternatively, was the stipulation made only after Rabban Shim'on ben Gamli'el stated his opinion? Or was it, for some reason, known only to Rabbi Yosi?