Just as we are used to sample variations between individual items if a product line, there are variations in the overall production process unless it is fully automated from source material to finished good and all of the facilities, tools, processes and conditions are fully optimized before mass production begins.
Especially under the varying workplace conditions caused by the pandemic and a production start in general, it would not surprise me if a single batch got out of spec for a reason that was only present temporarily as part of these changing conditions. Either it wasn't noticed or not thought to cause a problem significant enough to warrant a recall.
If you follow SpaceX development or just happened to pay attention to the launch of the Crew 1 mission (first US provided launch of humans to the ISS since the shuttle ended), you may have heard that a similar thing happened there. Even with the attention to detail in the rocket sector, a detail was missed. There was a variation in the effectiveness of a cleaning step performed by a subcontractor. On certain rocket engines, residue of the manufacturing process was not cleaned entirely, leading to blocked channels that affected engine startup reliability. Here's a nice quote from one of the guys looking into the matter, which illustrates my point:
“There’s certainly the possibility that we had cases of this earlier, and they were basically so harmless that we completely missed them,” Koenigsmann said. “It’s also possible that little things changed. This is a process that’s done out of house at a special vendor, so it could be that person is now more generous with cleaning fluids or other things.”
From:
https://spaceflightnow.com/2020/10/...-blocked-valve-ahead-of-november-crew-launch/