You can now extend a trimmed surface if Merge is turned off, and you select a boundary which is a surface isoparm (constant U or V). You cannot select a trim edge. The Extend tool is using the untrimmed version of the input surface, and creating an untrimmed surface.
In addition, if Merge is off, selecting an interior isoparm on a surface will now cause the extension to start from the selected isoparm, instead of from the closest end.
