Seeking doesn't display subtitles
When you're watching a movie with subtitles, afaik any format or container, if you seek then the subtitles at that point in time won't be shown. If you wish to see the subtitles to that part you've to seek back somewhen before the subtitles are triggered.
I dunno the internal details of VLC but it seems like all it needs to do is to check for the if some subtitle timestamp interval matches the new seek position and display it then.
For what it's worth, I've used VLC for nearly years and I think it always had this bug.