This is the killer feature. For videos with the same codec, Boilsoft uses a direct stream copy mode. It literally stitches the binary data together. This takes seconds, even for 4K files, and results in zero quality degradation.
Downloading version 9.1.9 from non-official sources carries real risks: Boilsoft Video Joiner 9.1.9 - ALLPCWorld
Solution: This happens with variable frame rate (VFR) videos from smartphones. Go to Settings > Advanced and enable "Re-encode audio" or "Fix audio sync." This is the killer feature