Adding an unused column creates an error in JoinAcross

Question

I'd been having trouble using JoinAcross on two datasets. After going through the answers to my original question and fixing my code, I find I'm now having a problem that I think is being caused by my data. I'm adding a column to an Excel sheet, but not using it in my dataset. I'm reading in two Excel sheets, "Reported" and "Estimated". Below are screens shot of "Reported" followed by "Estimated":

Next is my code and its output:

repFile = Import[NotebookDirectory[] <> "test.xlsx", {"Sheets", "Reported"}];
repHeader = repFile[[1]];
repData = repFile[[2 ;;]];

estFile = Import[NotebookDirectory[] <> "test.xlsx", {"Sheets", "Estimated"}];
estHeader = estFile[[1]];
estData = estFile[[2 ;;]];

repDS = Dataset@Map[AssociationThread[repHeader, #] &]@repData;
estDS = Dataset@Map[AssociationThread[estHeader, #] &]@estData;

rep1 = repDS[All, {"a", "z1"}];
est1 = estDS[All, {"a", "z2"}];

rep1
est1
rep1 // Normal
est1 // Normal
JoinAcross[rep1, est1, "a"]

Now I add one column to "Estimated" to get:

My output now looks like:

Since I'm not actually bringing in that additional column to the dataset I don't understand what's happening. Is this a bug or am I doing something wrong?