I’ve been meaning to raise this for ages, but I keep forgetting. 
On my Pi 3, if I whitelist 480p and 576p modes, and set “Settings/Player/Videos/Display 4:3 videos as” to “Normal”, then watch a 480i or 480p video, the aspect ratio gets messed up. If the video is 4:3 then it gets compressed horizontally; if the video is 16:9 then you end up with black bars top and bottom. 576 stuff gets messed up too.
This is easy enough to work around: if you set “Settings/Player/Videos/Display 4:3 videos as” to “Stretch 16:9” instead then all SD stuff gets played correctly, even including automatic aspect ratio switching (although 4:3 HD videos get stretched, of course). But the “Normal” option clearly shouldn’t be doing what it’s doing.