This has happened to me before.
You are using MS Edge, and probably copying and pasting from a text editor file, that was not saved as Rich Text Format (RTF).
If you use IE11, or Chrome browser instead you can copy & paste fine from any file format.
If you wish to use MS Edge, then before pasting into the forum click the icon at far right of toolbar that looks like a folder with wrench on it, then paste, then at bottom of the paste click the Use rich text editor.
If you are using Word or Wordpad, you could also save notes as a RTF file, then open the RTF file and copy & paste normally without needing to use the BB Code Editor icon.
This is a known issue and Ian, myself and others have used the Feedback tool in Windows 10 to ask Microsoft Developers to please restore RTE to the Edge Browser (RTE = Rich Text Editing).
Regards,
Regedit32