I'm not 100% sure if this is a bug in the way you describe it.
Perhaps you used "Custom position" once?
When using new subtitle parser/renderer there is a bug in thát option.
It is easy to accidentally click mouse pointer in the centre of the screen then (where the sample text is fixed/frozen).
See this bug-report:
http://forum.bsplayer.com/bug-report...html#post43364
But there is a bug anyhow!! (So I also linked to this post in my above mentioned bugs-summary).