I’ve actually mentioned this same thing to my athletes. Sometimes their unconcious takes over their body even when they feel like they should be putting forth more effort. Which seems to be very plausible. It’s even happened to me a few in the last 200m I ran while still competing. I ran a bad race in the 400m, and was dead after the race even though I ran a decent time. But later on in the day I had a 200m, and destroyed everyone in what felt like the easiest 200m of my life. It was at the time the 2nd fastest 200m I had ever run indoors.
I’ve never had anyone run pb or near pb’s en-route in a 400m.