When using properties mapper to change the text of a label, properties mapper uses the index of the label to determine which label to change. So, yes, you actually have to specify exactly which label to change.
With One-Click layout you cannot expect labels to be placed in a certain way. The algorithm will choose a layout strategy that is the most appropriate for the overall structure of the graph/diagram (according to its internal metrics). This may result in layout strategies that simply do not support label placement at all.
If you need control over label placement, use one of the layout algorithms that explicitly support (integrated) label placement or yEd's "Label Placement" algorithm.
In any case, be sure to set your edge label's "Preferred Placement" property to match the desired label placement. "Preferred Placement" tells an algorithm where to place labels (if it supports label placement), while "Placement" restrict the valid placement options. When placing labels with an algorithm, restricting valid placement options is not a good idea. In this case, use "SmartFree" or "Free" as "Placement".