Appending anything always goes in wrong spot
I consider this a bug although seems like it's an oversight.
When you want to add (insert or append) a feature like a measure or horizontal or vertical frame,
you define where to insert it by highlighting a measure as a reference point.
However it does not work as expected - while insertion does work and the feature inserted in front of the highlighted measure (not at the very beginning of the score), appending a feature does not append it
immediately after the highlighted measure, and always appends at the very end of the score.
This can be worked around by highlighting the "wrong" (next) measure and inserting in front of it, resulting in appending after the measure you really want to get appended after, but that's messy and requires editing measure's contents.
This has to be fixed.