Are you adding the tag using Opus or something else?
Are you in thumbnails mode when you make the change, or if not are any columns displaying which show metadata for video files? (e.g. duration, codec, dimensions, etc.)
After restarting Opus, is the tag there, or was it not saved?
It's possibly a bug in a video codec/splitter rather than in Opus itself. I've tried with a test MKV file here and didn't see any problems. If a crash dump exists matching the date & time of the crash, please zip & email it to leo@gpsoft.com.au as that may tell us which component is crashing.