Those are funny flight numbers for DL to use on this route. IIRC 3XXX usually means a RJ (Delta Connection carrier like SkyWest). Could these be KE codeshares, which would seem to be OK for leaving mainland China, but the reverse wouldn't work for entering China.

You might want to check also on the current rules for ICN transit.

ADDED: I just checked flight status on the DL app. DL 3460 says it's DL operated, so I'm now wondering whether it could be a ferry flight. It's showing on time, at 4:50 am-6:00 am ICN-PVG only. DL 284 is scheduled for PVG-ICN-DTW and left PVG at a totally different time, 11:14 am (delayed an hour and 9 minutes) with it also being delayed (an hour and 8 minutes) leaving ICN. It's already landed at DTW, only a half hour late.

DL 3461 SEA-ICN-PVG was all cancelled on December 28th (scheduled for 10:59 pm-3:50 am and 5:10 am-6:10 am). DL 288 (again totally different times) PVG-ICN-SEA was totally cancelled too. Scheduled times were 9:50 am-1:00pm and 2:50 pm - 8:10 am.