Like Orbitmic I got a large number of bags delayed, and that often happened on point to point flights and checking in plenty of time ahead.. In that case, the bag is not boarded on the aircraft and waits in some corner of the airport or is boarded on another flight. I still remember flying F class SFO-CDG with two bags checked in over 2 hours prior to departure. One bag flew with me and it turned out that another got stranded in SFO. As I had some VIP friends, the SFO station manager conducted an investigation and found my bag after a couple of days. it was randomly selected for a custom selection and put back in a wrong place where it sat for days. Not AF fault.

There is a distinct probability that your bag waited in ATL or was sent elsewhere. Being on different tickets increases the risk of mislabeling by the ATL checkin agent. An inexperienced domestic agent (you probably checked in domestic) can have trouble handling such a complex transaction and might have mislabeled involuntarily. And once the bag is stranded, it needs some detective work to link the tickets to final destination.

It might seem paradoxical, but a 5 hour connection in IAD might be too much. Airports do not have that much space and have different organizations. I do not know how IAD operates but it does not have a great reputation. If it arrived safely, your bag was probably transferred from domestic to international to a waiting area. It has to clear additional security and random custom inspection. Given the flight disruptions in USA and Europe in that period, the waiting area could have been quite messy.

Indeed, the bag might have been correctly labeled and flew with you to CDG. A 50min connection is likely to cause your bag taking a later flight. This is commonly handled at all airports including CDG, as long as you fly AF-AF. It would be the less likely scenario but there was a lot of weather disruptions in Europe causing some chaos.

Actually, a likely scenario is that AF agents did a good job tracking your bag somewhere in the world and trying to get it back to you speedily.

I truly sympathize with you and would have been upset too. But I also understand that bags delayed by a couple of days come with travel. Baggage handling is a very tough operation, even if some aspects have been automatized. Especially with long connection, an automated system will usually send the bag to the proper area, but then it gets handled by a human. In your itin, I would rate the risk well above 5%. Rather than trying to guess who is at fault in the long list of potential culprits, I have moved to a philosophical attitude and the use of carryons whenever I can.