"as if first entry  is a heading and the following are members of
that list"

This is exactly what is happening. In fact, I'm surprised you don't
get one repeated value every time. My only guess why would be that
the first item  in the list  is sometimes the only occurrence.

I have no idea what your spreadsheet looks like but I would try to
create a static heading field and then ignore the first cell of the
advanced filter output.

Yeah I understand the mess leading & trailing space can create.

I also have sidestepped the issue by keeping the first entry  as a heading &
ignoring it for my work. But I still feel I have not touched the real
cause of the problem. Let me encounter it once more and I will submit it
for your analysis.

Thanks for your time & effort.