The tk_textCopy (and tk_textCut) commands copy elided text from a text widget. While there are certainly some cases where this is desirable, I would imagine they are the minority. It's very minor to change the behaviour, just requires adding the -displaychars option to [$textWidget get].
The text man page, when describing the -elide option for tags, states that "Elided data (characters, images, embedded windows, etc) is not displayed and takes no space on screen, but further on behaves just as normal data."
My understanding is therefore that what you are observing is intentional, i.e. not a bug, although perhaps a questionable design.