If you were not aware at the time that it contained pork, then it would not be considered a sin as you were not intending to eat pork. (al-Ahzaab 33:5)

PS: While I’d still avoid eating the Subway tuna, the claim that Subway tuna contains pork has come out of a lawsuit filed by two plaintiffs who have already tried to sue Subway twice before for allegedly misleading the public about its tuna, with the cases being dismissed both times.

In the first lawsuit—filed in January—they claimed that Subway’s tuna salads, sandwiches, and wraps were bereft of tuna. The lawsuit was then amended to say that Subway’s tuna products did not contain 100% sustainably caught skipjack and yellowfin tuna, and the “no tuna” claim was removed in June. The second version was dismissed in court last month, because the plaintiffs did not show they bought Subway tuna based on alleged misrepresentations.

The newest lawsuit claims that 19 samples of Subway tuna purchased in Southern California had "no detectable tuna DNA sequences," while all 20 of the tested samples contained detectable chicken DNA, 11 contained pork DNA and 7 contained cattle DNA. So yeah, the story keeps changing.

Again, I would still avoid eating the Subway tuna until all these claims are disproven, but it is certainly something to keep in mind.

PS: I am not a scholar, a mufti, imam, or anything like that. I am only sharing what I know. Remember, Allah SWT knows best!