After testing some settings related to this post:
Loading Subtitles & Maximizing Causes Terrible Screen Lags
it seems that i found a bug in new subtitle rendering engine.
Loaded an MKV file with an external srt file. Positioned subtitles close to the bottom of the video window. After going to full screen, subtitles are partialy on the black border added and not at the bottom of the image. Unchecking "enable drawing on black border...." generated another strange thing. Subtitles are now displayed somwhere around 1/3 of the image when going full screen. Using internal renderer, subtitles goes partialy on the black border.
Using old engine, no such problem.