windows 10 64bwhich OS are you running?
also desyncs the audio and video, maybe it's problem of recording with quick sync :SAvidemux is what I use. It allows you to cut your video without re-encoding.
windows 10 64b
I've used Avidemux with good results for cutting segments of .mp4s for later highlight reel creation. I've always saved out of OBS as .mp4 though, not .flv. Never noticed an audio desync issue.