I'm not sure it's a bug but it feels that way.
When the black lines are activated (so that the subs can go below the video) and I switch to fullscreen and back, SMPlayer always skips the minimum seek interval. In my case I've set that interval to be 5 seconds, so when I switch between fullscreen and window mode, the program jumps 5 seconds forward. Happens every time. I even set the minimum interval to 36 minutes (just for an experiment) and when I switched to fullscreen, SMPlayer skipped 36 minutes forward.
Mint 18.3 Cinnamon x64.
Player version: 15.11.0
Using Qt 5.5.1 (compiled with Qt 5.5.1)
Using MPlayer SVN r0