I just hope you’re clear that the facetious comments about paranormal activity don’t mean that we’re taking your problem lightly, it’s just that an explanation for what you’ve experienced is truly elusive!
The only way that you should be able to have the items you’ve cooked come out of the bath already overcooked is for the temperature of the bath to have increased above the 134F temperature at which you had your Anova set. Lack of water circulation, double layering the food in the bags, even leaving too much air in the bags (which we’ve already ruled out) can cause a problems - but the problems don’t include overcooking. You’ve also confirmed that the temp of the water is correct using a reliable external thermometer so, at least when you check it now, it is heating correctly.
As I mentioned in my previous comment, a temporary spike in temperature could yield an overcooked product, though I don’t know of anything that might cause such a temporary spike. I wouldn’t rule out the possibility entirely but if such a spike were to occur it also seems unlikely that it would only spike by a handful of degrees F. It’s especially weird that we see items ranging from medium to well-done all in the same cook bath. By definition, every item cooked in that bath should be cooked to the same degree of “doneness”.
Is there any chance that the quality of the meats itself might be suspect? Not sure why this might make them seem more “done” than expected, but just the fact they differed in how cooked they were coming out of the same bath suggests differences between the meats.
How about the searing? I know you said that you only seared them for about a minute on each side…but is there any chance you misjudged and seared overlong? (I’m speaking from experience here one time when I wasn’t paying enough attention!)
Is there any chance you have a remaining steak from that same batch that has not yet been cooked? Maybe you can bag it up and step by step go through a perfect sous vide cook (flawless bagging, ensuring good circulation during cook, checking temp a few times during the cook with external thermometer, etc.) - then check the doneness prior to searing when you take it out of the bath. This is what I think I might try just to see if the meat might be “odd”. Kind of grasping at straws here - this is a real conundrum!
Whatever you do please let me know. Fingers crossed for ya!
