Since we can't see your unit or the complex you live in, there is the idea (and it is only one possibility) that ALL of the units were installed with improperly sized line sets.
Or, there is another idea that the line set was recommended for replacement because of a refrigerant leak (which is never "seen" in the conventional sense of the term) that was determined to be in the line set. This happens when the system loses a charge, but the exposed parts of the system do not present a leak. Then, the default answer is the leak is in a portion of a system that cannot be seen or tested. that would be the line set.
Let us know what your second opinion reveals.