If you don't know, you can check:
(rightclick >) Options > Preferences > Subtitles [Subtitle properties]
If "Use new subtitle parser/renderer" isn't selected/checked you're using "old" subs-engine.
I also think that's still default (untill all bugs related to subs and new engine have been solved).
However, I meanwhile also tried with old subtitle engine and couldn't reproduce your experience.
Can you yourself reproduce it? (Could you also test with new subs-engine please?)
Can any other user with Windows 7 64 bit please also test this?