Hi Drew,
I've found out what it's happening. The data frame is ED50, but my service is ETRS89, and when you load my service in Arcmap, it doesn't apply any transformation by default (see attached screenshot1). However if I select the correct transformation method the location is fine (see attached screenshot2, we have a customized transformation that is more accurate, but you can test with ED_1950_To_ETRS_1989_7). I don't know how difficult is you include that in your widget, but I think it should be able to specify a coordinate system transformation method. What do you think of about?
Thanks very much,
Pedro.