Given that my cases were in 2017, no, they were not doing COVID tested flights in 2017, so I'm not sure why you brought that up/mentioned it/jumped to that conclusion. My case simply had to do with, for whatever reason in SEA, when I took the SEA-BOS/BOS-AMS-BIO trip, the agent in SEA was unable to check me in for BOS-AMS-BIO. I had to check-in and get my boarding passes in BOS. But my bag was tagged through to BIO when I checked-in in SEA. Conversely, earlier that year, I was able to be checked in for a flight on a separate ticket (on a partner no less) with no issue. So I was simply providing two examples of how it could go either way for OP where he/she may or may not be able to check-in for JFK-MXP in DFW. I've even had similar issues on domestic DL to DL trips on separate tickets where an agent at my first origin was unable to check me in for the second portion of the trip but was able to tag bags through to my true final destination.