Are you using JRVR? Subtitle moving should be working relatively consistently for it. madVR does the moving itself, we have no influence on it, and with RO Standard it'll probably not work.
JRVR will also try to generally match subtitles to a full 16:9 frame, even if its cropped, so moving should typically not be required, no matter if a video includes black bars, or was cropped.
In general, only plain text subtitles can be moved. Bitmap subtitles, or complex text subtitles (ASS) cannot be moved reliably.
If you are using JRVR and its not rendering them to the bottom of the frame, a sample might be useful to see whats going on exactly.
I've also already made some changes for the next build to ensure the subtitle renderer is always working with the correct information on the window size and video size. I'm not certain if that was an issue, but its definitely more correct this way.