I don't know about the Memphis or Atlanta contracts, but I'm fairly certain that SD and NZP had specific guarantees on length of stay each time the contract was re-negotiated, for both adults and potential or existing cubs. Sometimes, the original duration of a cub's stay, usually three years, has been extended by China; Shi Shi was returned but only in exchange for Gao Gao. China hasn't just suddenly called for a return before; all pandas to my knowledge have stayed the full duration of their contract.
That's what makes this so shocking. The fact that SD immediately announced plans to renegotiate illustrates how clearly they want these animals and would not have favored this return. I can only guess that SD's contract had expired and that legally, China could claim no agreement had been broken. Xao Li Wu was here longer than any other cub, far exceeding three years. Perhaps the last contract had expired with some tacit understanding that Mr. Wu would be called back eventually, but Bai Yun and Gao Gao would live out their days here. If a written legal contract had been breached, I think SD would have made some kind of stink instead of seeming so obedient and subservient. It's impossible though to think of this move as anything but retaliation for the tariffs. I'm sure no one anywhere thinks it ideal to subject elderly ànimals at the very upper limits of their life expectancy to long overseas travel in a shipping crate they clearly had no time to (re)adapt to.