On December 22nd, DL 287 SEA-ICN-PVG turned around after crossing the Pacific, returned to SEA, and cancelled the flight. DL 288 PVG-ICN-SEA was also cancelled on December 25th. There's a thread on this in the DL forum.

Passengers were apparently told that China made some changes to requirements for PVG while the flight was in the air. The announcement vaguely suggested that passengers would be unable to enter China. A later announcement claimed that the aircraft would have been required to spend more time on the ground at PVG, thus making the flight and its schedule infeasible.

Has anyone in China heard about this? What specific changes were made to the rules?

SPECULATION:

My guess is that perhaps all crew are now required to be tested in China even though they don't leave the aircraft in PVG. (Apparently the flight stops in ICN to get fresh crew and to refuel, with no passengers permitted to get on or off there.) If a crew member would test positive, my guess would be that the quarantine and testing requirements for all passengers would be much harsher. Perhaps the crew member(s) would be required to stay in China for a couple weeks or until testing negative.

Alternatively, perhaps China is no longer permitting foreign carriers to operate this type of flight versus being forced to fly nonstop from their country of origin. If the change is the latter, it might impact HKG too, where some airlines now are stopping elsewhere in Asia and then flying short legs to/from HKG to avoid the possibility of crew being sent to quarantine centers there.

I guess there could be a requirement to fumigate the aircraft before boarding starts in PVG; this would also lead to additional time on the ground at PVG.