The issue is that one data table has your column as a character and the other table the column is numeric. If you attempt to concatenate to the data table that is numeric, and you specify "Append to first table", the data from the second table will be set to missing values, if their values can not convert to numeric. If the table you concatenate to is has the column as a character, the values will be conerted.
I tested this in both JMP 15 and JMP 17 and both versions had the same results.
Jim