i have been having this issue in the prior versions, too. never brought it up thoug... but maybe i am just missing a checkbox or such...
i have a layout in which there is a window seperated in 2 parts: on the left, there is the editor, on the right there is the live viewer.
now, if i choose to work in a vertical format like, let's say, 720*1280 or such, the live viewer looses it's format/proportions easily.
i made some screenshots, showing the progression, beginning with a very "thin" lv-format, which seems to show the frame proportions correctly, indicated by the red horizontal lines.
but the more i widen the lv-window, the more it looses it's proportions. i hope the screenshots show what i mean in a clearer manner:
