Domino supports 2 ways of storing the contents of a 'Rich Text' item in a Notes Document.
- Domino's Native Rich Text format
- MIME / Html - Which is used throughout the internet and mail systems.
There is a setting on the Item Properties which determines what format
'Store contents as Mime/html'.
The XPages control only supports MIME / HTML.
When you load a document's Rich Text Item that is stored as the Native Rich Text Format, it converts it from Native to MIME / HTML upon loading the document so that it can be used with the XPages control.
This explains why pasting it into the document and then loading via XPages works, as you are pasting in the Native rich text, but it converts it when opening.
If you try to copy from Notes Client (Native Format) to XPages it will only support whatever format that CKEditor will accept for pasting and I don't think Domino Native Rich Text is one of them.
This is still the same whether using Xpinc or Web browser
If you are going to be doing anything fancy with XPages Rich Text it is worth learning about MIME and how it works as it will save you some headaches.