yes, that's quite strange looking at first glance. To me, it looks like expanding the node, i.e. the pure visibilty of the dimension affects the column segmentation. Well, reading the HELP about the above functions and other inter chart record functions, I think it should never return you a result, since Location and Time are giving you unique combinations for all rows, building new column segments, right?
(You could see what I am talking about by adding an expression =rowno() and playing around)
I think a workaorund for your needs might be to add an aggr around the above, like
=aggr(above([Event Time]),Location,[Event Time])
Hope this helps,