Tricky detective work
Just to illustrate the sort of thing that can go wrong (this is NOT a suggestion of what has actually gone wrong), just suppose that the temperature data was output fom the batteries in deg F, when it should have been output in deg C, and suppose that 10 deg C was the minimum operating temperature.
Then the temperature control would be happy to keep the battery above 10 deg F, equal to minus 12 deg C, and the data logged would "prove" that the temperature never went below the limit of 10 degrees.
Of course it will not turn out to be anything that simple, but it is an example of how the cause of a problem can sometimes hide the evidence.