This is "TinyMCE issue". It is slightly better with newer versions (so are you using 5.0.4?), but there are lots of issues with WSIWG editors since they need to cleanup garbage formating from external sources like MS Word for instance. As for the CTRL+V vs paste dialog, thats because of a issue in Gecko (Firefox), as it does not fire any event for the editor to listen to on keyboard / mouse paste.
Andre, thanks for your reply and sorry for delayed answer. The strange thing is I have already tested this with the original TinyMCE at their homepage with the demo they have - the same text is parsed absolutely correctly. Also, I have in usage TinyMCE implemented into a standalone extension for xmlblock datatype, and the paste there is also correct with the same text.
