Marker error, and youtube chapters

You need to provide exact steps and specify whether Ripple Markers is turned on. If it is, this might be by design.

What you wrote is not very understandable. Also, steps to reproduce it are needed. Think: how can I make this a useful bug report instead of vague descriptions of moments of confusion or surprise?

  1. I would like to consider overlap of markers an error

Not a bug

  1. File->export->markers… do not generate the correct file

Yes it does, but it does not export markers with a duration > 1 by design (documented here). The format does not support duration and typically these are used for special purpose like export.