Yesterday I've bought the full version of OPlayer, but it has the same problem
How to reproduce:
1. Open and watch movie with external .srt subtitles
2. Set 5 second for the Rewind Time (in settings)
3. Try to repeat several times (on the same position) - press Rewind Back button or use geisture to swipe from right to left. After several attempts, subtitles disappear. It it is needed to repeat several times to become them visible again.
this bug is very uncomfortoble for me and I hope it will be fixed soon. Thank you.