This has bugged me forever (AutoCAD 2.6). It's been ignored for far too long and needs to be fixed.
Stretching is way too picky about viewports. If you cross-select part of an object and start to stretch, you cannot change viewports or the selection set is ignored. I, just now, tried to stay in the same viewport (because I know), but I rolled my mouse-wheel to get closer in and the orignal part of the mesh I was stretching went off the bottom of the screen. You guessed it....my selection was lost.
Way so picky? Once you drag a selection and have the vert sub objects defined, it shouldn't matter if I finish in another viewport, or zoom in and move around in the same viewport. Sometimes the thing you are stretching has to be selected in a macro view (of a particular viewport) but the placement needs to happen zoomed in to see what you are doing (or from another viewport altogether).
It would be awesome if this limitation that plagued the Stretch command since the dawn of time, would go away. While you are at it, make a "previous" option to stretch automatically reselect the previous stretch elements. That would be sweet.
Since you say it's done this since R2.6, I have to assume you're not talking about MViews in layouts. Which leads to the old VPORTS command.
I just did it in 2012.
Don Ireland
Engineering Design Technician