The freeze on the $400 billion 25-year deal was not due to U.S. sanctions; Chinese SOEs have never cared about U.S. sanctions, and the Chinese government surely took U.S. sanctions into account when it signed the deal. The real reason is Iran's failure to honor the prerequisite terms of the 25-year deal.
First of all, the prerequisite for China to provide armaments was for the Iranian military to completely abandon GPS in favor of China's Beidou, but Iran has not done so. Secondly the prerequisite for the $400 billion investment was ports and oil traded in CNY, but Iran gave the ports to India and only part of the oil trade was switched to CNY.
the Iranian government is in chaos internally, and there are very large forces that are preventing cooperation with China. It is difficult for China and Iran to cooperate until Iran cleans up the internal surrenderists and Mossad.