Dekel, thank you for revealing more details. Unfortunately they don't add much clarity to identifying the cause of the observed overdonesness. Most of the evidence you present leads to the expectation that you would have had undercooked steaks as your result.
You appear to have a good understanding of the essentials of SV cooking as demonstrated by your awareness of temperature as it relates to doneness, thickness being a critical factor, and how time affects tenderness.
There is one particularly glaring earlier omission, - "completely still water". Does your Nano circulate the water when operating?
To ensure even heat penetration, particularly in an overcrowded container, water must circulate continuously throughtout while cooking.
Anytime in the future should you have a reoccurance of this apparant overcooking problem please immediately use a probe thermometer to ascertain the meat's internal temperature. We humans can often be mislead by our senses, but thermometers are rarely very wrong.
To simplify future problem solving opportunities you may have i recommend cooking significantly different cuts of meat separately. To my taste the Picanha requires about double the cooking time of a rib eye steak of equal thickness.
In your transition to SV cooking you will achieve greater succees if you break with much of your prior experience and knowledge of conventional cooking techniques. SV cooking presents you with different observable outcomes that may lead you to making errors.
Some examples are:
Meat coloration can be an unreliable indicator of doeness when cooked in the absence of air. Verify internal temperature with a thermometer.
Low temperature cooking results in significantly less moisture loss in meat giving it a more solid feel to the touch than conventionally cooked meat. I think of it being more plump. Many cooks use the somewhat subjective pinch test to monitor doneness. I prefer the precision of a thermometer.
So we still have your mystery cook that apparently defies logic and the laws of thermodynamics. Steaks cooked at 134ᴼF, at the high end of Medium-Rare, resulted in steaks that appeared to be cooked at a much higher temperature.
It is a mystery.
Have you experienced any other potentially para-normal events in your kitchen.