As the title says. In subtitleeditor video subtitles are not shown during video playback, using either the internal or external (mplayer) video player. It’s a known bug in current upstream gstreamermm version (0.10.11).
Workaround for oS 12.3 64-bit: downgrade to libgstreamermm2-0.10.10.2 from oS 12.2. This rpm can be found at achaios build repo (thanks achaios :-)):
Note: not tested in oS 13.1.